Democratic peace theory

Democracies have been defined differently by different theorists and researchers; this accounts for some of the variations in their findings. Some examples:

Small and Singer define democracy as a nation that (1) holds periodic elections in which the opposition parties are as free to run as government parties, (2) allows at least 10% of the adult population to vote, and (3) has a parliament that either controls or enjoys parity with the executive branch of the government.^[28]

Doyle requires (1) that "liberal regimes" have market or private property economics, (2) they have policies that are internally sovereign, (3) they have citizens with juridical rights, and (4) they have representative governments. Either 30% of the adult males were able to vote or it was possible for every man to acquire voting rights as by attaining enough property. He allows greater power to hereditary monarchs than other researchers; for example, he counts the rule of Louis-Philippe of France as a liberal regime.^[29]

レイは、成人人口の少なくとも50%が投票権を持ち、選挙によって独立した政党から別の政党へ行政権が平和的かつ憲法に基づいて少なくとも1回移譲されたことを要件としている。この定義では、民主主義的とみなされることが多い長期間は除外される。例えば、1800年までのアメリカ合衆国、独立から1979年までのインド、1993年までの日本はすべて一党支配体制であったため、この定義ではカウントされない。^{[ 30 ]}

ランメルは、「民主主義とは、権力を持つ者が秘密投票と幅広い選挙権（成人男性の少なくとも3分の2を含むと大まかに理解される）による競争選挙で選出される自由民主主義を意味し、言論、宗教、結社の自由があり、政府が従属し平等な権利を保障する憲法上の法体系が存在する」と述べている。^{[ 31 ]}

国際戦争に関する定量的研究では、通常、戦争を1年間に戦闘で1000人以上が死亡した軍事紛争と定義しています。これは、戦争に関する多くの研究にデータを提供してきた「戦争相関プロジェクト」で使用されている定義です。問題となっている軍事紛争のほとんどは、この閾値を明らかに上回るか下回ることが判明しています。^{[ 44 ]}

研究者によって異なる定義が用いられている。例えば、ウィアートは戦争を200人以上の戦闘死者が出るものと定義している。^{[ 36 ]}ラセットは古代ギリシャを考察する際、両陣営に国家の認可を受けた軍隊が関与する実際の戦闘のみを戦争の定義としている。^{[ 45 ]}

戦争相関プロジェクトの分類では、軍事化された国家間紛争（MID）は戦争よりも小規模な紛争です。このような紛争は、戦闘による死者を伴わない単なる軍事力の誇示に過ぎない場合もあります。MIDと戦争を合わせて「軍事化された国家間紛争」またはMICと呼びます。MIDには戦争に先行する紛争も含まれるため、MIDとMICの違いは見た目ほど大きくないかもしれませんが、これはまだ調査が必要な分野です。 ^{[ 46 ]}

Statistical analysis and concerns about degrees of freedom are the primary reasons for using MID's instead of actual wars. Wars are relatively rare. An average ratio of 30 MIDs to one war provides a richer statistical environment for analysis.^[47]

Most research is regarding the dyadic peace, that democracies do not fight one another. Very few researchers have supported the monadic peace, that democracies are more peaceful in general. There are some recent papers that find a slight monadic effect. Müller and Wolff, in listing them, agree "that democracies on average might be slightly, but not strongly, less warlike than other states," but general "monadic explanations are neither necessary nor convincing." They note that democracies have varied greatly in their belligerence against non-democracies.^[48]

One example from the first group is that liberal democratic culture may make the leaders accustomed to negotiation and compromise. Policy makers who have built their careers within a political culture of non-violent accommodations with domestic rivals, unlike autocrats who typically hold power through the threat of coercion, will be inclined toward non-violent methods abroad.^[36][48] Another that a belief in human rights may make people in democracies reluctant to go to war, especially against other democracies. The decline in colonialism, also by democracies, may be related to a change in perception of non-European peoples and their rights.^[80]

Bruce Russett also argues that the democratic culture affects the way leaders resolve conflicts. In addition, he holds that a social norm emerged toward the end of the nineteenth century; that democracies should not fight each other, which strengthened when the democratic culture and the degree of democracy increased, for example by widening the franchise. Increasing democratic stability allowed partners in foreign affairs to perceive a nation as reliably democratic. The alliances between democracies during the two World Wars and the Cold War also strengthened the norms. He sees less effective traces of this norm in Greek antiquity.^[81]

Hans Köchler relates the question of transnational democracy to empowering the individual citizen by involving him, through procedures of direct democracy, in a country's international affairs, and he calls for the restructuring of the United Nations Organization according to democratic norms. He refers in particular to the Swiss practice of participatory democracy.^[24]

Mousseau argues that it is market-oriented development that creates the norms and values that explain both democracy and the peace. In less developed countries individuals often depend on social networks that impose conformity to in-group norms and beliefs, and loyalty to group leaders. When jobs are plentiful on the market, in contrast, as in market-oriented developed countries, individuals depend on a strong state that enforces contracts equally. Cognitive routines emerge of abiding by state law rather than group leaders, and, as in contracts, tolerating differences among individuals. Voters in marketplace democracies thus accept only impartial ‘liberal’ governments, and constrain leaders to pursue their interests in securing equal access to global markets and in resisting those who distort such access with force. Marketplace democracies thus share common foreign policy interests in the supremacy—and predictability—of international law over brute power politics, and equal and open global trade over closed trade and imperial preferences. When disputes do originate between marketplace democracies, they are less likely than others to escalate to violence because both states, even the stronger one, perceive greater long-term interests in the supremacy of law over power politics.^[82][83]

Braumoeller argues that liberal norms of conflict resolution vary because liberalism takes many forms. By examining survey results from the newly independent states of the former Soviet Union, the author demonstrates that liberalism in that region bears a stronger resemblance to 19th-century liberal nationalism than to the sort of universalist, Wilsonian liberalism described by democratic peace theorists, and that, as a result, liberals in the region are more, not less, aggressive than non-liberals.^[77]

A 2013 study by Jessica Weeks and Michael Tomz found through survey experiments that the public was less supportive of war in cases involving fellow democracies.^[84]

The case for institutional constraints goes back to Immanuel Kant, who wrote:^[85]

[I]f the consent of the citizens is required in order to decide that war should be declared (and in this constitution it cannot but be the case), nothing is more natural than that they would be very cautious in commencing such a poor game, decreeing for themselves all the calamities of war. Among the latter would be: having to fight, having to pay the costs of war from their own resources, having painfully to repair the devastation war leaves behind, and, to fill up the measure of evils, load themselves with a heavy national debt that would embitter peace itself and that can never be liquidated on account of constant wars in the future.

Democracy thus gives influence to those most likely to be killed or wounded in wars, and their relatives and friends (and to those who pay the bulk of the war taxes.)^[86] This monadic theory must, however, explain why democracies do attack non-democratic states. One explanation is that these democracies were threatened or otherwise were provoked by the non-democratic states. Doyle argued that the absence of a monadic peace is only to be expected: the same ideologies that cause liberal states to be at peace with each other inspire idealistic wars with the illiberal, whether to defend oppressed foreign minorities or avenge countrymen settled abroad.^[87] Doyle also notes liberal states do conduct covert operations against each other; the covert nature of the operation, however, prevents the publicity otherwise characteristic of a free state from applying to the question.^[88]

Charles Lipson argues that four factors common in democracies give them a "contracting advantage" that leads to a dyadic democratic peace: (1) Greater transparency, (2) Greater continuity, (3) Electoral incentives for leaders to keep promises, and (4) Constitutional governance.^[89]

Studies show that democratic states are more likely than autocratic states to win the wars that they start. One explanation is that democracies, for internal political and economic reasons, have greater resources. This might mean that democratic leaders are unlikely to select other democratic states as targets because they perceive them to be particularly formidable opponents. One study finds that interstate wars have important impacts on the fate of political regimes, and that the probability that a political leader will fall from power in the wake of a lost war is particularly high in democratic states.^[41]

ゲルピとグリースドルフが述べたように、いくつかの研究では、自由主義指導者は、幅広い利害関係者の同意なしに戦争のために国家資源を動員する能力を阻害する制度化された制約に直面していると主張している。^{[ 76 ]}ソ連の後継国家の市民とエリートの態度を比較した調査結果は、この主張と一致している。^{[ 77 ]}さらに、これらの制約は他国には容易に明らかであり、指導者が操作することはできない。したがって、民主主義国家は、武力行使への嫌悪感を他国に伝える信頼できるシグナルを送っている。これらのシグナルにより、民主主義国家は互いに紛争を避けることができるが、非民主主義国家からの攻撃を招く可能性がある。民主主義国家は、そのような攻撃に対して、おそらく先制的に、武力行使によって対応するよう圧力を受ける可能性がある。また、ゲルピとグリースドルフが述べたように、民主主義国の指導者が国際危機をエスカレートさせることを選択した場合、その脅しは非常に信頼できるものと受け止められるという研究結果もある。なぜなら、そのような行動には比較的大きな世論が存在するはずだからである。自由主義国家間の紛争では、交渉シグナルの信頼性によって、動員前に平和的な解決を交渉することができる。^{[ 76 ]}ジェフ・カーターによる2017年の研究では、民主主義国家は戦争への動員が遅いという証拠が見つかった。^{[ 90 ]}

上記2つと同様のゲーム理論に基づく説明としては、国民の参加と公開討論によって、民主主義国家の意図に関する明確で信頼できる情報が他国に伝わるというものがある。対照的に、非民主主義国家の指導者の意図、譲歩がどのような影響を与えるか、約束が守られるかどうかを知ることは難しい。したがって、紛争当事者の少なくとも一方が非民主主義国家である場合、不信感が生じ、譲歩を渋ることになる。^{[ 91 ]}

The risk factors for certain types of state have, however, changed since Kant's time. In the quote above, Kant points to the lack of popular support for war – first that the populace will directly or indirectly suffer in the event of war – as a reason why republics will not tend to go to war. The number of American troops killed or maimed versus the number of Iraqi soldiers and civilians maimed and killed in the American-Iraqi conflict is indicative. This may explain the relatively great willingness of democratic states to attack weak opponents: the Iraq war was, initially at least, highly popular in the United States. The case of the Vietnam War might, nonetheless, indicate a tipping point where publics may no longer accept continuing attrition of their soldiers (even while remaining relatively indifferent to the much higher loss of life on the part of the populations attacked).

Coleman uses economic cost-benefit analysis to reach conclusions similar to Kant's. Coleman examines the polar cases of autocracy and liberal democracy. In both cases, the costs of war are assumed to be borne by the people. In autocracy, the autocrat receives the entire benefits of war, while in a liberal democracy the benefits are dispersed among the people. Since the net benefit to an autocrat exceeds the net benefit to a citizen of a liberal democracy, the autocrat is more likely to go to war. The disparity of benefits and costs can be so high that an autocrat can launch a welfare-destroying war when his net benefit exceeds the total cost of war. Contrarily, the net benefit of the same war to an individual in a liberal democracy can be negative so that he would not choose to go to war. This disincentive to war is increased between liberal democracies through their establishment of linkages, political and economic, that further raise the costs of war between them. Therefore, liberal democracies are less likely to go war, especially against each other. Coleman further distinguishes between offensive and defensive wars and finds that liberal democracies are less likely to fight defensive wars that may have already begun due to excessive discounting of future costs.^[92]

Brad LeVeck and Neil Narang argue that democratic states are less likely to produce decision-making errors in crises due to a larger and more diverse set of actors who are involved in the foreign policy decision-making process.^[93]

Using selectorate theory, Bruce Bueno de Mesquita, James D. Morrow, Randolph M. Siverson and Alastair Smith argue that the democratic peace stems in part from the fact that democratic leaders sustain their power through large winning coalitions, which means that democratic leaders devote more resources to war, have an advantage in war, and choose wars that they are highly likely to win. These leads democratic states to avoid one another, but war with weak non-democratic states.^[94]

A prominent rational choice argument for the democratic peace is that democracies carry greater audience costs than authoritarian states, which makes them better at signaling their intentions in interstate disputes.^[95][96] Arguments regarding the credibility of democratic states in disputes has been subject to debate among international relations scholars. Two studies from 2001, using the MID and ICB datasets, provided empirical support for the notion that democracies were more likely to issue effective threats.^[97][76] However, a 2012 study by Alexander B. Downes and Todd S. Sechser found that existing datasets were not suitable to draw any conclusions as to whether democratic states issued more effective threats.^[10] They constructed their own dataset specifically for interstate military threats and outcomes, which found no relationship between regime type and effective threats.^[10] A 2017 study which recoded flaws in the MID dataset ultimately conclude, "that there are no regime-based differences in dispute reciprocation, and prior findings may be based largely on poorly coded data."^[10] Other scholars have disputed the democratic credibility argument, questioning its causal logic and empirical validity.^[98] Research by Jessica Weeks argued that some authoritarian regime types have similar audience costs as in democratic states.^[99][100]

A 2021 study found that Americans perceived democracies to be more likely to back down in crises, which contradicts the expectations of the audience costs literature.^[101]

さまざまな説明の研究を促す一般的な批判の一つは、実際にはこの理論は「民主主義が平和をもたらす」と主張することはできないという点である。なぜなら、一般的に民主主義がより平和であるという証拠は非常に少ないか、あるいは存在しないからである。この理論は「共同民主主義が平和をもたらす」という主張しか支持できない。ロザートによれば、これは民主主義が実際に原因であるかどうかに疑問を投げかける。なぜなら、もしそうであれば、単独の効果が期待されるからである。^{[ 9 ]}

こうした認識されている異常に対する最も単純な説明（ただし、リアリストのロザートが好むものではない。下記のリアリストの説明の項を参照）は、民主主義国家同士が平和であるのは、民主主義国家だからではなく、民主主義スコアが似ているからだというものである。この考え方は、「独裁的平和」効果、つまり、両方とも非民主的、あるいは両方とも非常に非民主的な国家間の戦争の確率が低下するという、いくつかの独立した観察から始まった（もちろん、どの著者もその存在を否定していない）。^{[ 102 ] [ 103 ]}これにより、民主的平和は、実際には類似している国家のサブセットを分析する際に、特殊なケースとして現れるという仮説が立てられた。^{[ 104 ]}あるいは、一般的に類似性は戦争の確率にのみ影響するのではなく、完全な民主主義や厳格な独裁制といった強力な政治体制の一貫性にのみ影響するという仮説である。

Autocratic peace and the explanation based on democratic similarity is a relatively recent development, and opinions about its value are varied. Henderson builds a model considering political similarity, geographic distance and economic interdependence as its main variables, and concludes that democratic peace is a statistical artifact which disappears when the above variables are taken into account.^[105] Werner finds a conflict reducing effect from political similarity in general, but with democratic dyads being particularly peaceful, and noting some differences in behavior between democratic and autocratic dyads with respect to alliances and power evaluation.^[104] Beck, King, and Zeng use neural networks to show two distinct low probability zones, corresponding to high democracy and high autocracy.^[106][d] Petersen uses a different statistical model and finds that autocratic peace is not statistically significant, and that the effect attributed to similarity is mostly driven by the pacifying effect of joint democracy.^[107] Ray similarly disputes the weight of the argument on logical grounds, claiming that statistical analysis on "political similarity" uses a main variable which is an extension of "joint democracy" by linguistic redefinition, and so it is expected that the war reducing effects are carried on in the new analysis.^[108] Bennett builds a direct statistical model based on a triadic classification of states into "democratic", "autocratic" and "mixed". He finds that autocratic dyads have a 35% reduced chance of going into any type of armed conflict with respect to a reference mixed dyad. Democratic dyads have a 55% reduced chance. This effect gets stronger when looking at more severe conflicts; for wars (more than 1000 battle deaths), he estimates democratic dyads to have an 82% lower risk than autocratic dyads. He concludes that autocratic peace exists, but democratic peace is clearly stronger. However, he finds no relevant pacifying effect of political similarity, except at the extremes of the scale.^[109]

To summarize a rather complex picture, there are no less than four possible stances on the value of this criticism:

1. Political similarity, plus some complementary variables, explains everything. Democratic peace is a statistical artifact. Henderson subscribes to this view.
2. Political similarity has a pacifying effect, but democracy makes it stronger. Werner would probably subscribe to this view.
3. Political similarity in general has little or no effect, except at the extremes of the democracy-autocracy scale: a democratic peace and an autocratic peace exist separately, with the first one being stronger, and may have different explanations. Bennett holds this view, and Kinsella mentions this as a possibility
4. Political similarity has little or no effect and there is no evidence for autocratic peace. Petersen and Ray are among defendants of this view.

One study has argued that there have been as many wars between democracies as one would expect between any other couple of states. Its authors conclude that the argument for democratic peace "rests in an ambiguity", since empirical evidence not confirm neither deny democratic pacifism, and strongly relies upon what degree of democracy makes a government democratic; according to them "because perfect democracy is infeasible, one can always sidestep counter-evidence by raising the bar of democracy".^[113]

民主的平和の証拠はいくつかあるかもしれないが、データサンプルや期間が小さすぎて決定的な結論を導き出すことはできないと述べる人もいる。^{[ 114 ] [ 59 ] [ 28 ]}例えば、Gowaは、民主主義国の数が少なすぎるため、1939年以前の民主的平和の証拠は重要ではないと判断し、それ以降の期間については別の現実主義的な説明を提示している。 ^{[ 59 ]} Gowaの統計の使用は批判されており、他のいくつかの研究やレビューでは異なる、あるいは反対の結果が得られている。^{[ 76 ] [ 25 ]}しかし、これはこの理論に対する最も長く続いている批判と見なすことができる。前述のように、支持者の中にも、少なくとも全面戦争だけを考慮すると、その妥当性を評価するための統計サンプルは限ら​​れているか、不足していることに同意する人がいる。

民主主義と戦争についてかなり限定的な定義を用いたある研究によると、1816年から1992年の間に、共同で民主主義国家である2つの国家間で戦争は発生しなかった。国家間の戦争が民主主義的性格に関係なく完全にランダムに分布すると仮定すると、民主主義国家間の紛争の予測数は約10になる。したがって、レイは、この証拠は統計的に有意であると主張するが、将来、少数の民主主義国家間の戦争でも、このような証拠を打ち消すことは依然として考えられる。^{[ 25 ] [ e ]}

領土平和理論は、民主主義が平和をもたらすよりも、平和が民主主義をもたらすという主張である。この主張は、平和がほぼ常に民主主義に先行し、国境紛争がすべて解決されるまで国家は民主主義を発展させないことを示す歴史的研究によって裏付けられている。これらの研究は、平和が民主主義を引き起こすという強い証拠があるが、民主主義が平和を引き起こすという証拠はほとんどないことを示している。^{[ 69 ]}

平和が民主主義を生み出すという仮説は、心理学および文化理論によって裏付けられています。クリスチャン・ウェルツェルの人間エンパワーメント理論は、実存的安全保障が解放的な文化的価値観と民主的な政治組織への支持につながると主張しています。^{[ 115 ]}これは、進化心理学に基づくいわゆる王権理論からも導き出されます。

領土平和論は、隣国と紛争状態にある国が民主主義を発展させる可能性が低い理由を説明する。民主的平和論は、隣国ではない国間の平和や、既に平和な関係にある国同士の関係により関連している。^{[ 116 ]}

他のいくつかの理論では、省略された変数が平和と民主主義の両方を説明すると主張している。民主主義と平和の両方を説明する可能性のある変数には、制度、商業、相互依存、同盟、米国の世界支配、政治的安定性などがある。^{[ 7 ] [ 8 ] [ 9 ] [ 10 ]}

これらの理論については、「その他の説明」の項でさらに詳しく説明します。

民主的平和論の批判者たちは、自由主義国家はしばしば「ならず者国家」「破綻国家」「悪」とみなす非自由主義国家と紛争を起こすと指摘している。^{[ 117 ]}いくつかの研究では、非民主主義国家との戦争を含めた場合、民主主義国家が独裁国家よりも戦争を起こす可能性が低いことを裏付けることはできていない。^{[ 118 ]}

エドワード・ギボンは、ローマ人の主要な征服は共和政時代に達成されたことを強調し、皇帝たちは概して、元老院の政策、執政官の積極的な競争、そして民衆の軍事的熱意によって獲得された領土を維持することに満足していたと述べた。^{[ 119 ]}

民主主義の方が意図をより信頼性高く示すことができるという考えには異論がある。^{[ 10 ]}

一部の著者は、国家が経済や安全保障上の懸念など、自国の客観的な利益や動機の結果として他国の政体タイプを絶えず再解釈していると主張して、民主主義の定義を批判している。^{[ 9 ]}例えば、ある研究では、19 世紀末には西側のオピニオン リーダーはドイツを民主主義国家とみなしていたが、第一次世界大戦前の数年間、米国、フランス、英国との関係が悪化し始めると、実際の政体変更がないにもかかわらず、ドイツは徐々に独裁国家として再解釈されるようになったと報告している。^{[ 120 ] [ 121 ]}シミンは、1989 年から 1999 年にかけてのミロシェヴィッチのセルビアに対する西側の認識に関して同様の批判をしている。^{[ 122 ]}ルメルはこの批判に対し、一般的に民主的平和に関する研究は他国の民主主義に対する認識に焦点を当てていないと述べている。そしてセルビアの具体的なケースでは、1990年代初頭に西側民主主義国がミロシェヴィッチに与えた限定的な評価は民主主義の承認ではなく、代替の指導者がさらに悪いかもしれないという認識に過ぎない、と主張している。^{[ 123 ]}

民主的平和論の研究者の中には、同様のデータセット全体を検証・修正することなく、特定の紛争を非戦争、あるいは政治体制を非民主主義と事後的に再分類したとして批判されている者もいる。民主的平和論の支持者と反対者は、修正の妥当な根拠があったとしても、これは統計の不適切な使用であるという点で一致している。 ^{[ 22 ] [ 58 ] [ 59 ]}アジア・タイムズ紙の軍事コラムニストは、上記の批判をジャーナリスト風に要約し、この理論は「真のスコットランド人」問題に陥っていると述べている。つまり、例外は「真の」民主主義国家間や「真の」戦争間ではないと説明されているのだ。^{[ 124 ]}

民主主義的平和論の研究者の中には、大統領が実質的に争われた選挙の結果選出されることを要件とする者もいる。これは限定的な定義かもしれない。例えば、米国国立公文書館は、「事実上、ジョージ・ワシントンは1789年と1792年の両年において、大統領選挙で対立候補がいなかった」と記している。（選挙人団の当初の規定では、大統領と副大統領への投票に区別はなく、各選挙人は2人の異なる候補者に投票する必要があり、次点者が副大統領となることになっていた。すべての選挙人がワシントンに1票を投じ、^{[ 125 ]}ジョン・アダムズは残りの票の過半数を獲得した。他にも複数の候補者がいたため、副大統領選挙は争われた。）

スピロは、使用された統計的手法について他にもいくつかの批判を行った。^{[ 114 ]}ラセットとレイが記述した一連の論文は、例えば異なる方法論を用いてこれに応答した。^{[ 126 ] [ 25 ]}

使用されたデータセット自体が批判されることもある。例えば、一部の著者は、特に内戦において、戦闘による死者数に民間人の死者が含まれていないことを「戦争相関データ」が批判している。コーエンとウィークスは、死者が出ず、暴力の脅威も一般的に非常に限定的な漁業紛争のほとんどは、軍事紛争のリストからも除外されるべきだと主張している。^{[ 127 ]}グレディッチは「戦争相関データ」データセットに対していくつかの批判を行い、改訂版データセットを作成した。^{[ 62 ]}マオズとラセットは「ポリティI」と「ポリティII」データセットに対していくつかの批判を行ったが、それらは後のバージョンでほとんど対処されている。^{[ 23 ]}

最も包括的な批判では、「民主主義」はめったに定義されず、実質的な民主主義を指すことはなく、因果関係が不明確であり、100以上の研究で反駁され、約200の逸脱事例を説明できず、ある国が海外に民主主義を拡大しようとすることを正当化するためにイデオロギー的に推進されてきたと指摘している。^{[ 128 ]}ほとんどの研究は、「民主主義」という複雑な概念を次元化しようとするのではなく、二変量変数として扱っている。また、研究は、民主主義には数十種類あるという事実を考慮に入れていないため、特定の種類の民主主義に明確に示さない限り、または異なるデータセットを使用して、協調型民主主義や経済民主主義など、すべての種類に当てはまると主張されない限り、結果は無意味である。

民主主義規範の説明に関する最近の研究は、この説明の根拠となるミクロ的基盤が経験的に裏付けられていないことを示している。これまでの研究のほとんどでは、民主主義社会における自由主義規範の存在と、それが戦争を遂行する意思に及ぼす影響は、単に仮定されただけで、測定されたことはなかった。さらに、これらの規範が他の政体タイプに存在しないかどうかは調査されなかった。最近の2つの研究は、自由主義規範の存在を測定し、これらの規範が戦争を遂行する意思に及ぼすと想定される影響を調査した。両方の研究の結果は、自由民主主義規範は自由民主主義国家だけでなく、他の政体タイプにも存在することを示している。さらに、これらの規範は、戦争の瀬戸際にある国家間紛争中に他国を攻撃する意思には影響しない。^{[ 129 ] [ 130 ]}

セバスチャン・ロサトは、民主的平和論にはいくつかの誤った前提があると論じている。第一に、民主的な国民は戦争の代償に否定的に反応すると想定している。しかし、現代の戦争では死傷者は比較的少なく、兵士の多くは志願兵であり、戦闘のリスクを受け入れているため、兵士の死の代償を最も重く受ける家族や友人は、徴兵された兵士の家族や友人よりも政府を批判する可能性が低い。第二に、民主的平和論はナショナリズムの役割を無視している。民主的な国民も他の人々と同じようにナショナリズム感情の影響を受けやすく、もし民主的な国民が戦争が国家にとって必要だと信じるならば、国民はそれを支持するだろう。最後に、民主的な指導者は世論に従うのと同じくらい、世論を誘導する可能性が高い。民主的な指導者はナショナリズム感情の力を認識していることが多く、そのため戦争に関しては、戦争は国家の生活様式を守ったり広めたりするために必要だと主張し、ナショナリズム感情を煽ろうとする。この点において、民主主義の指導者は権威主義者よりも優位に立つ可能性さえある。なぜなら、彼らはより正当に国民を代表していると見なされるからである。ロザートは、これは防衛戦争だけでなく侵略戦争にも当てはまると主張している。民主主義の国民は、侵略戦争が国益にかなうと見なされれば、ナショナリズム感情に駆り立てられてそれを支持することができる。^{[ 9 ]}

Rosato also argues that authoritarian leaders have a reduced incentive to go to war because civilian control over the military is less guaranteed in autocracies; there is always the risk the military could subvert civilian leadership and a war which results in defeat could swiftly result in a coup. Even military dictators run the risk of internal dissent within the armed forces. Autocratic leaders in general also risk unleashing political and social turmoil that could destroy them if they go to war. Conversely, bellicose democratic leaders can rely on the acknowledgement of the legitimacy of the democratic process, as pacifist actors in democracies will need to respect the legitimacy of a democratically elected government. If pro-war groups can capture the organs of the state in a democracy legitimately, then anti-war groups will have little means of opposing them outside of extra-constitutional means, which would likely backfire and cause the anti-war groups to lose legitimacy.^[9]

A 2017 study found that public opinion in China showed the same reluctance in going to war as publics in democratic states, which suggests that publics in democratic states are not generally more opposed to war than publics in authoritarian states.^[131]

The peacefulness may have various limitations and qualifiers and may not actually mean very much in the real world.

Democratic peace researchers do in general not count as wars conflicts which do not kill a thousand on the battlefield; thus they exclude for example the bloodless Cod Wars. However, research has also found a peacefulness between democracies when looking at lesser conflicts.

Liberal democracies have less of these wars than other states after 1945. This might be related to changes in the perception of non-European peoples, as embodied in the Universal Declaration of Human Rights.^[80]

Related to this is the human rights violations committed against native people, sometimes by liberal democracies. One response is that many of the worst crimes were committed by non-democracies, like in the European colonies before the nineteenth century, in King Leopold II of Belgium's privately owned Congo Free State, and in Joseph Stalin's Soviet Union. The United Kingdom abolished slavery in British territory in 1833, immediately after the Reform Act 1832 had significantly enlarged the franchise. (Of course, the abolition of the slave trade had been enacted in 1807; and many DPT supporters would deny that the UK was a liberal democracy in 1833 when examining interstate wars.)

Hermann and Kegley Jr. argue that interventions between democracies are more likely to happen than projected by an expected model.^[132] They further argue that democracies are more likely to intervene in other liberal states than against countries that are non-democracies.^[133] Finally, they argue that these interventions between democracies have been increasing over time and that the world can expect more of these interventions in the future.^{[132][133][134]} The methodology used has been criticized and more recent studies have found opposing results.^[62]

Rummel argues that the continuing increase in democracy worldwide will soon lead to an end to wars and democide, possibly around or even before the middle of this century.^[135] The fall of Communism and the increase in the number of democratic states were accompanied by a sudden and dramatic decline in total warfare, interstate wars, ethnic wars, revolutionary wars, and the number of refugees and displaced persons.^[136] One report claims that the two main causes of this decline in warfare are the end of the Cold War itself and decolonization; but also claims that the three Kantian factors have contributed materially.^[137]

Economic historians Joel Mokyr and Hans-Joachim Voth argue that democratic states may have been more vulnerable to conquest because the rulers in those states were too heavily constrained. Absolutist rulers in other states could, however, operate more freely.^[138]

Adamson et al (2025) finds evidence against democratic peace theory in the ancient world, with evidence instead indicating more intense conflicts between two polities when both were democratic in ancient Greece (Athens-Syracuse) and the Mediterranean (Roman Republic-Carthage) and that ancient democracies were also more belligerent in general. They argue this suggests that democracy's relationship with peace is contingent rather than inherent.^[139] Eric Robinson makes a similar proposition, arguing that Greek democracies fought each other even outside of the Athens-Syracuse rivalry (such as in Greek Sicily) and thus democratic peace in the modern day may be due to contextual or structural differences between ancient and modern democracies.^[140]

民主的平和理論の批判者たちは、民主主義国家間の秘密作戦や軍事介入を指摘し、これらの介入は民主主義国家が必ずしも互いを信頼し尊重しているわけではないことを示していると主張している。^{[ 9 ]}アレクサンダー・B・ダウンズとラリー・ローレン・リリーは、民主主義国家が行う秘密作戦は、どのバージョンの民主的平和理論に従うかによって意味合いが異なると主張している。彼らは、秘密作戦は規範と抑制と均衡を重視する民主的平和理論の変種とは矛盾するが、秘密作戦は選択理論の大規模勝利連合と小規模勝利連合の概念に依拠する民主的平和理論の変種とはより整合的である可能性があると主張している。^{[ 141 ]}

マイケル・ポズナンスキーによる2015年の研究は、民主主義国家が互いに秘密裏に介入しているという調査結果を、民主主義国家は他国の民主主義的性格が崩壊または衰退すると予想したときにそうするのだと主張することで調和させている。^{[ 142 ]}

2022年の研究では、民主主義国が他の民主主義国に対して代理戦争を行うことは稀であることが判明した。「強力な民主主義制度は、選出された指導者が姉妹政権に対して代理戦争を行うことを阻止し、禁輸違反は民主主義制度が弱い場合に発生する傾向がある」^{[ 143 ] 。}

チャイム・カウフマンは、イラク戦争に至るまでの経緯は、民主主義国家における戦争の制約は、民主主義政府が情報を統制・操作し、政権のレトリックに反する諜報調査結果を抑圧できるかどうか、また強力な野党や影響力のあるメディアが存在するかどうかにかかっていることを示していると主張している。^{[ 144 ]}

多くの民主主義国家は、戦争によって非民主主義国家へと変貌する。侵略される側、あるいは侵略する側（クーデター直後など）として戦争に巻き込まれる場合があり、時にはクーデターの指導者が意図的に戦争を引き起こしたこともある。

カール・シュミットは憲法を覆す方法について「例外を決定する者が主権者である」と書いた。^{[ 145 ]}シュミットは、指導者以外に誰も信用しないよう国民を説得するのに役立つため、国内（および国外）の敵が必要であることについて、再び次のように述べている。「国家が政治的実体である限り、国内の平和に対するこの要求は、危機的な状況において国内の敵についても決定することを国家に強いる。したがって、すべての国家は、国内の敵を宣言するための何らかの公式を提供する。」 いかなる反対勢力も、実際の外国の敵の傀儡として描かれ、意図されることになる。^{[ 146 ]}

資本主義的平和、あるいは資本主義的平和理論は、経済発展（資本主義）に関する所定の基準に従えば、先進国同士は戦争を起こさず、小規模な紛争もほとんど起こらないと主張する。これらの理論は、民主主義と民主主義国家間の平和の両方を説明することで、民主的平和の根拠として提唱されてきた。因果関係の正確な性質は、提案された変数と、その概念の指標として用いられる尺度によって異なる。

民主主義の決定要因に関する研究者の大多数は、経済発展が安定した健全な民主主義の形成を可能にする主要な要因であるという点で一致している。^{[ 147 ] [ 148 ]}そのため、一部の研究者は、経済発展が平和の確立にも影響を与える要因であると主張している。

Mousseau argues that a culture of contracting in advanced market-oriented economies may cause both democracy and peace. These studies indicate that democracy, alone, is an unlikely cause of the democratic peace. A low level of market-oriented economic development may hinder development of liberal institutions and values.^{[82][149][150][83]} Hegre and Souva confirmed these expectations.^[151][152] Mousseau finds that democracy is a significant factor only when both democracies have levels of economic development well above the global median. In fact, the poorest 21% of the democracies studied, and the poorest 4–5% of current democracies, are significantly more likely than other kinds of countries to fight each other.^[83] Mousseau, Hegre, and Oneal confirm that if at least one of the democracies involved has a very low level of economic development, democracy is ineffective in preventing war; however, they find that when also controlling for trade, 91% of all the democratic pairs had high enough development for the pacifying effect of democracy to be important during the 1885–1992 period and all in 1992.^[153] The difference in results of these two studies may be due to sampling: Mousseau's 2005 study observed only neighboring states where poor countries actually can fight each other. In fact, fully 89% of militarized conflicts between less developed countries from 1920 and 2000 were among directly contiguous neighbors.^[154] He argues that it is not likely that the results can be explained by trade: Because developed states have large economies, they do not have high levels of trade interdependence.^[155][156] In fact, the correlation of developed democracy with trade interdependence is a scant 0.06 (Pearson's r – considered substantively no correlation by statisticians.^[157])

Both World Wars were fought between countries which can be considered economically developed. Mousseau argues that both Germany and Japan – like the USSR during the Cold War and Saudi Arabia today – had state-managed economies and thus lacked his market norms.^[158] Hegre finds that democracy is correlated with civil peace only for developed countries, and for countries with high levels of literacy. Conversely, the risk of civil war decreases with development only for democratic countries.^[147]

Gartzke argues that economic freedom (a quite different concept from Mousseau's market norms) or financial dependence explains the developed democratic peace, and these countries may be weak on these dimensions too.^{[159][160][161]} Rummel criticizes Gartzke's methodology and argues that his results are invalid.^[162]

Allan Dafoe, John R. Oneal, and Bruce Russett have challenged Gartzke and Mousseau's research.^[163]

Several studies find that democracy, more trade causing greater economic interdependence, and membership in more intergovernmental organizations reduce the risk of war. This is often called the Kantian peace theory since it is similar to Kant's earlier theory about a perpetual peace; it is often also called "liberal peace" theory, especially when one focuses on the effects of trade and democracy. (The theory that free trade can cause peace is quite old and referred to as Cobdenism.) Many researchers agree that these variables positively affect each other but each has a separate pacifying effect. For example, in countries exchanging a substantial amount of trade, economic interest groups may exist that oppose a reciprocal disruptive war, but in democracy such groups may have more power, and the political leaders be more likely to accept their requests.^{[164][165][166]} Weede argues that the pacifying effect of free trade and economic interdependence may be more important than that of democracy, because the former affects peace both directly and indirectly, by producing economic development and ultimately, democracy.^[148] Weede also lists some other authors supporting this view. However, some recent studies find no effect from trade but only from democracy.^[167][168]

None of the authors listed argues that free trade alone causes peace. Even so, the issue of whether free trade or democracy is more important in maintaining peace may have potentially significant practical consequences, for example on evaluating the effectiveness of applying economic sanctions and restrictions to autocratic countries.

It was Michael Doyle who reintroduced Kant's three articles into democratic peace theory. He argued that a pacific union of liberal states has been growing for the past two centuries. He denies that a pair of states will be peaceful simply because they are both liberal democracies; if that were enough, liberal states would not be aggressive towards weak non-liberal states (as the history of American relations with Mexico shows they are). Rather, liberal democracy is a necessary condition for international organization and hospitality (which are Kant's other two articles)—and all three are sufficient to produce peace.^[29][169] Other Kantians have not repeated Doyle's argument that all three in the triad must be present, instead stating that all three reduce the risk of war.

Immanuel Wallerstein has argued that it is the global capitalist system that creates shared interests among the dominant parties, thus inhibiting potentially harmful belligerence.^[170]

Toni Negri and Michael Hardt take a similar stance, arguing that the intertwined network of interests in the global capitalism leads to the decline of individual nation states, and the rise of a global Empire which has no outside, and no external enemies. As a result, they write, "The era of imperialist, interimperialist, and anti-imperialist wars is over. (...) we have entered the era of minor and internal conflicts. Every imperial war is a civil war, a police action".^[171]

Many studies supporting the theory have controlled for many possible alternative causes of the peace. Examples of factors controlled for are geographic distance, geographic contiguity, power status, alliance ties, militarization, economic wealth and economic growth, power ratio, and political stability. These studies have often found very different results depending on methodology and included variables, which has caused criticism. DPT does not state democracy is the only thing affecting the risk of military conflict. Many of the mentioned studies have found that other factors are also important.^{[41][108][166]}

Several studies have also controlled for the possibility of reverse causality from peace to democracy. For example, one study supports the theory of simultaneous causation, finding that dyads involved in wars are likely to experience a decrease in joint democracy, which in turn increases the probability of further war. So they argue that disputes between democratizing or democratic states should be resolved externally at a very early stage, in order to stabilize the system.^[172] Another study finds that peace does not spread democracy, but spreading democracy is likely to spread peace. A different kind of reverse causation lies in the suggestion that impending war could destroy or decrease democracy, because the preparation for war might include political restrictions, which may be the cause for the findings of democratic peace.^[173] However, this hypothesis has been statistically tested in a study whose authors find, depending on the definition of the pre-war period, no such effect or a very slight one. So, they find this explanation unlikely.^[47] This explanation would predict a monadic effect, although weaker than the dyadic one.

Weart argues that the peacefulness appears and disappears rapidly when democracy appears and disappears. This in his view makes it unlikely that variables that change more slowly are the explanation.^[36] Weart, however, has been criticized for not offering any quantitative analysis supporting his claims.^[174]

Wars tend very strongly to be between neighboring states. Gleditsch showed that the average distance between democracies is about 8000 miles, the same as the average distance between all states. He believes that the effect of distance in preventing war, modified by the democratic peace, explains the incidence of war as fully as it can be explained.^[58]

A 2020 study in International Organization found that it was not democracy per se that reduces the prospects for conflict, but whether women's suffrage was ensured. The study argued, "women's more pacific preferences generate a dyadic democratic peace (i.e., between democracies), as well as a monadic peace."^[175]

According to Azar Gat's War in Human Civilization, there are several related and independent factors that contribute to democratic societies being more peaceful than other forms of governments:^[176]

1. 富と快適さ：民主主義社会における繁栄の増大は平和と結びついてきた。なぜなら、市民は前近代よりも家庭での生活がより豊かになったため、戦争や兵役の苦難に耐えることをあまり望まなくなったからである。富の増大は快適さを通じて戦争を減少させる働きをしてきた。^{[ 177 ]}
2. 都市型サービス社会：軍隊の新兵の大多数は農村部出身者か工場労働者である。多くの人は、こうした人々が戦争に適していると考えている。しかし、技術が進歩するにつれて、軍隊はコンピュータ化されたデータに依存する高度な情報サービスへと移行し、都市部の人々がこのサービスに多く採用されるようになった。^{[ 178 ]}
3. 性革命：ピルの普及と女性の労働市場への参入により性行為の機会が増えたことが、男性の戦争への意欲低下につながったもう一つの要因である可能性がある。若い男性は、人生の楽しみを捨てて軍隊の厳しさと貞操を守ることに抵抗を感じるようになっている。^{[ 179 ]}
4. 若い男性の減少：平均寿命が延びたことで、若い男性の数が減少しています。若い男性は最も攻撃的で、軍隊に入隊する割合も最も高いです。先進社会では若い男性の数が少ないため、平和性が高まっていることが説明できるかもしれません。^{[ 180 ]}
5. 家族あたりの子供の数が少ない（出生率が低い）：前近代では、家族にとって子供を失うことは常に辛いことでしたが、現代では、1人か2人の子供しか持たない家族が増えたため、より困難になっています。親が戦争で子供を失うリスクを冒すことはさらに難しくなっています。しかし、ガットはこの議論は難しいものであると認識しています。なぜなら、前近代では子供の平均寿命が長くなく、大家族が必要だったからです。^{[ 181 ]}
6. 女性の参政権：女性は男性ほど露骨に攻撃的ではない。そのため、女性は深刻な暴力に走る傾向が少なく、男性ほど暴力を支持しない。自由民主主義国家では、女性は選挙で選ばれることで政府に影響を与えることができた。より多くの女性を選出することは、自由民主主義国家が特定の問題に対してより積極的なアプローチを取るかどうかに影響を与える可能性がある。^{[ 182 ]}
7. 核兵器：核兵器は、大国間の戦争が起こらない理由の一つかもしれない。核戦争は相互確証破壊（MAD）につながると多くの人が考えている。MADとは、核戦争に関与する両国が、両陣営が全滅するまで相手を攻撃できる能力を持つことを意味する。そのため、各国は全滅を恐れて相手を攻撃したがらない。^{[ 183 ]}

Supporters of realism in international relations in general argue that not democracy or its absence, but considerations and evaluations of power, cause peace or war. Specifically, many realist critics claim that the effect ascribed to democratic, or liberal, peace, is in fact due to alliance ties between democratic states which in turn are caused, one way or another, by realist factors.

For example, Farber and Gowa find evidence for peace between democracies to be statistically significant only in the period from 1945 on, and consider such peace an artifact of the Cold War, when the threat from the communist states forced democracies to ally with one another.^[184] Mearsheimer offers a similar analysis of the Anglo-American peace before 1945, caused by the German threat.^[185] Spiro finds several instances of wars between democracies, arguing that evidence in favor of the theory might be not so vast as other authors report, and claims that the remaining evidence consists of peace between allied states with shared objectives. He acknowledges that democratic states might have a somewhat greater tendency to ally with one another, and regards this as the only real effect of democratic peace.^[114] Rosato argues that most of the significant evidence for democratic peace has been observed after World War II; and that it has happened within a broad alliance, which can be identified with NATO and its satellite nations, imposed and maintained by American dominance as part of Pax Americana. One of the main points in Rosato's argument is that, although never engaged in open war with another liberal democracy during the Cold War, the United States intervened openly or covertly in the political affairs of democratic states several times, for example in the Chilean coup of 1973, the Operation Ajax (1953 coup in Iran) and Operation PBSuccess (1954 coup in Guatemala); in Rosato's view, these interventions show the United States' determination to maintain an "imperial peace".^[9]

The most direct counter arguments to such criticisms have been studies finding peace between democracies to be significant even when controlling for "common interests" as reflected in alliance ties.^[76][25] Regarding specific issues, Ray objects that explanations based on the Cold War should predict that the Communist bloc would be at peace within itself also, but exceptions include the Soviet Invasion of Afghanistan, the Cambodian-Vietnamese War, and the Sino-Vietnamese War. Ray also argues that the external threat did not prevent conflicts in the Western bloc when at least one of the involved states was a non-democracy, such as the Turkish Invasion of Cyprus (against Greek Junta supported Cypriot Greeks), the Falklands War, and the Football War.^[41] Also, one study notes that the explanation "goes increasingly stale as the post-Cold War world accumulates an increasing number of peaceful dyad-years between democracies".^[186] Rosato's argument about American dominance has also been criticized for not giving supporting statistical evidence.^[187]

Some realist authors also criticize in detail the explanations first by supporters of democratic peace, pointing to supposed inconsistencies or weaknesses.

Rosato criticizes most explanations to how democracy might cause peace. Arguments based on normative constraints, he argues, are not consistent with the fact that democracies do go to war no less than other states, thus violating norms preventing war; for the same reason he refutes arguments based on the importance of public opinion. Regarding explanations based on greater accountability of leaders, he finds that historically autocratic leaders have been removed or punished more often than democratic leaders when they get involved in costly wars. Finally, he also criticizes the arguments that democracies treat each other with trust and respect even during crises; and that democracy might be slow to mobilize its composite and diverse groups and opinions, hindering the start of a war, drawing support from other authors.^[9] Another realist, Layne, analyzes the crises and brinkmanship that took place between non-allied democratic great powers, during the relatively brief period when such existed. He finds no evidence either of institutional or cultural constraints against war; indeed, there was popular sentiment in favor of war on both sides. Instead, in all cases, one side concluded that it could not afford to risk that war at that time, and made the necessary concessions.^[188]

Rosato's objections have been criticized for claimed logical and methodological errors, and for being contradicted by existing statistical research.^[27] Russett replies to Layne by re-examining some of the crises studied in his article, and reaching different conclusions; Russett argues that perceptions of democracy prevented escalation, or played a major role in doing so.^[126] Also, a recent study finds that, while in general the outcome of international disputes is highly influenced by the contenders' relative military strength, this is not true if both contenders are democratic states; in this case the authors find the outcome of the crisis to be independent of the military capabilities of contenders, which is contrary to realist expectations.^[76] Finally, both the realist criticisms here described ignore new possible explanations, like the game-theoretic one discussed below.^[189]

A different kind of realist criticism stresses the role of nuclear weapons in maintaining peace. In realist terms, this means that, in the case of disputes between nuclear powers, respective evaluation of power might be irrelevant because of Mutual assured destruction preventing both sides from foreseeing what could be reasonably called a "victory".^[190] The 1999 Kargil War between India and Pakistan has been cited as a counterexample to this argument,^[61] though this was a small, regional conflict and the threat of WMDs being used contributed to its de-escalation.^[191]

Some supporters of the democratic peace do not deny that realist factors are also important.^[126] Research supporting the theory has also shown that factors such as alliance ties and major power status influence interstate conflict behavior.^[25]

Many researchers have reacted to this limitation by studying lesser conflicts instead, since they have been far more common. There have been many more MIDs than wars; the Correlates of War Project counts several thousand during the last two centuries. A review lists many studies that have reported that democratic pairs of states are less likely to be involved in MIDs than other pairs of states.^[25]

Another study finds that after both states have become democratic, there is a decreasing probability for MIDs within a year and this decreases almost to zero within five years.^[196]

When examining the inter-liberal MIDs in more detail, one study finds that they are less likely to involve third parties, and that the target of the hostility is less likely to reciprocate, if the target reciprocates the response is usually proportional to the provocation, and the disputes are less likely to cause any loss of life. The most common action was "Seizure of Material or Personnel".^[192]

Studies find that the probability that disputes between states will be resolved peacefully is positively affected by the degree of democracy exhibited by the lesser democratic state involved in that dispute. Disputes between democratic states are significantly shorter than disputes involving at least one undemocratic state. Democratic states are more likely to be amenable to third party mediation when they are involved in disputes with each other.^[25]

In international crises that include the threat or use of military force, one study finds that if the parties are democracies, then relative military strength has no effect on who wins. This is different from when non-democracies are involved. These results are the same also if the conflicting parties are formal allies.^[76] Similarly, a study of the behavior of states that joined ongoing militarized disputes reports that power is important only to autocracies: democracies do not seem to base their alignment on the power of the sides in the dispute.^[197]

Some fear that the democratic peace theory may be used to justify wars against non-democracies in order to bring lasting peace, in a democratic crusade.^[209]Woodrow Wilson in 1917 asked Congress to declare war against Imperial Germany, citing Germany's sinking of American ships due to unrestricted submarine warfare and the Zimmermann telegram, but also stating that "A steadfast concert for peace can never be maintained except by a partnership of democratic nations" and "The world must be made safe for democracy."^[210][g]R. J. Rummel was a notable proponent of war for the purpose of spreading democracy, based on this theory.

Some point out that the democratic peace theory has been used to justify the 2003 Iraq War, others argue that this justification was used only after the war had already started.^[211] Furthermore, Weede has argued that the justification is extremely weak, because forcibly democratizing a country completely surrounded by non-democracies, most of which are full autocracies, as Iraq was, is at least as likely to increase the risk of war as it is to decrease it (some studies show that dyads formed by one democracy and one autocracy are the most warlike, and several find that the risk of war is greatly increased in democratizing countries surrounded by non-democracies).^[148] According to Weede, if the United States and its allies wanted to adopt a rationale strategy of forced democratization based on democratic peace, which he still does not recommend, it would be best to start intervening in countries which border with at least one or two stable democracies, and expand gradually. Also, research shows that attempts to create democracies by using external force has often failed. Gleditsch, Christiansen, and Hegre argue that forced democratization by interventionism may initially have partial success, but often create an unstable democratizing country, which can have dangerous consequences in the long run.^[62] Those attempts which had a permanent and stable success, like democratization in Austria, West Germany and Japan after World War II, mostly involved countries which had an advanced economic and social structure already, and implied a drastic change of the whole political culture. Supporting internal democratic movements and using diplomacy may be far more successful and less costly. Thus, the theory and related research, if they were correctly understood, may actually be an argument against a democratic crusade.^{[36][39][211]}

Michael Haas has written perhaps the most trenchant critique of a hidden normative agenda. Among the points raised: Due to sampling manipulation, the research creates the impression that democracies can justifiably fight non-democracies, snuff out budding democracies, or even impose democracy. And due to sloppy definitions, there is no concern that democracies continue undemocratic practices yet remain in the sample as if pristine democracies.^[212]

This criticism is confirmed by David Keen who finds that almost all historical attempts to impose democracy by violent means have failed.^[213]

Republican liberalism is a variation of Democratic Peace Theory which claims that liberal and republican democracies will rarely go to war with each other. It argues that these governments are more peaceful than non-democracies and will avoid conflict when possible. According to Micheal Doyle, there are three main reasons for this: Democracies tend to have similar domestic political cultures, they share common morals, and their economic systems are interdependent.^[214][215] Liberal democracies (republics) that trade with each other, are economically dependent on one another and therefore, will always attempt to maintain diplomatic relations as to not disrupt their economies.

Liberalism, as an overarching theory, holds that diplomacy and cooperation is the most effective way to avoid war and maintain peace.^[216] This is contrasting to the theory of realism, which states that conflict will always be recurrent in the international system, whether due to human nature or the anarchic international system.^[217]

The concept of Republican liberalism is thought to have initially originated from Immanuel Kant's book "Perpetual Peace: A Philosophical Sketch" (1795). The term "Perpetual Peace" refers to the permanent establishment of peace, and was made notorious by the book. Democratic peace, commercial peace and institutional peace were all advanced in the book as well. It takes a rather utopian view, that humanities' desire for peace will out compete humanities' desire for war.^[218]

Kant and the liberal school of thought view international co-operation as a more rational option for states than resorting to war. However, the neo-liberal approach concedes to the realist school of thought, that when states cooperate it is simply because it is in their best interest. Kant insisted that a world with only peace was possible, and he offered three definitive articles that would create the pathway for it. Each went on to become a dominant strain of post–World War II liberal international relations theory.^[219]

I: "The Civil Constitution of Every State should be Republican"

Kant believed that every state should have Republican style form of government. As in, a state where "supreme power is held by the people and their elected representatives."^[220] Kant saw this in Ancient Rome, where they began to move away from Athenian democracy (direct democracy) and towards a representative democracy. Kant believed giving the citizens the right to vote and decide for themselves would lead to shorter wars and less wars. He also thought it was important to "check the power of monarchs",^[221] to establish a system of checks and balances where no one person holds absolute power. Peace is always dependent on the internal character of governments. Republics, with a legislative body that will be able to hold the executive leader in check and maintain the peace.

II: "The Law of Nations shall be founded on a Federation of Free States"

Kant argues nations, like individuals can be tempted to harm each other at any given moment. So, rule of law should be established internationally. Without international laws and courts of judgement, then force would be the only way to settle disputes. States ought to instead develop international organisations and rules that facilitate cooperation. In any case, some kind of federation is necessary in order to maintain peace between nations. Contemporary examples include the United Nations and the European Union, which try to maintain peace and encourage cooperation among nations.

III: "The Law of World Citizenship shall be Limited to Conditions of Universal Hospitality"

Kant is referring to "the right of the stranger not to be treated as an enemy when he arrives in the land of another.".^[218] So long "stranger" is peaceful, he should not be treated with hostility. However, this is not the right to be a "permanent visitor", simply as a temporary stay. This is applicable in the contemporary world when a country is receiving a world leader. The host country usually holds a state welcoming ceremony which strengthens diplomatic relations.

There is significant debate over whether the lack of any major European general wars since 1945 is due to cooperation and integration of liberal-democratic European states themselves (as in the European Union or Franco-German cooperation), an enforced peace due to the intervention of the Soviet Union and the United States until 1989 and the United States alone thereafter,^[222] or a combination of both.^[223] The debate over this theory was thrust in the public eye when the 2012 Nobel Peace Prize was awarded to the European Union for its role in creating peace in Europe.^[224] Notable major wars in Europe after 1945 are Yugoslav Wars and Russian invasion of Ukraine, which follows the prediction of the interactive model of democratic peace.^[110]