As culture grows and men gradually move towards greater agreement over their principles, they lead to mutual understanding and peace.”
The theory of democratic peace is perhaps one of the most widely accepted propositions among international relations scholars today. A vast body of literature, from theoretical elaborations to statistical measurements, concerning liberal peace and/or democratic peace theory has been developed and has explored the proposition profoundly.
The theoretical background dates back to Immanuel Kant’s writings in the 18th century, especially to the ideas in his “Perpetual Peace” treatise, but with the late 1980s – the end of the Cold War – an explosion of scholarly interest in the topic of democratic peace has taken place. Although there exists a dissenting minority, the consensus among the great majority of scholars claims that democratic states/liberal democracies do not or are “less likely to fight wars with each other”.
“[The] absence of war between democratic states comes as close as anything we have to an empirical law in international relations.”
Recently, this opinion or discussion enhanced its scope. It left the dimension of academia while reaching the policy-level of Western countries, namely the U.S..
But, although wars between democracies are seemingly absent, this does not mean that liberal states are totally reluctant to use military force or to fight wars. Concerning this it is to say that the idea of democratic peace does not only include the thesis that wars between democracies are less/not likely to appear.
Since the substantial scientific discussion concerning democratic peace has accelerated in the late 1980s/early 1990s, four other characteristics of the democratic peace have been formulated.
First, proponents of the democratic peace argue that democracies tend to win through wars they fight with nondemocracies. Second, some scholars pointed out that democracies suffer less casualties and fight shorter in wars that are initialized by them, compared with non-democratic states. Third, it has been underlined that liberal democracies which face serious disputes with democratic peers opt for more peaceful or mitigating means of conflict resolution than other (non-democratic) dyads in conflict. Fourth and finally, scholars like Randall Schweller brought forward the argument that powerful democracies do not engage in preventive wars.
While it is almost empirically proven that the probability of wars between democratic states is very low or even zero, war is obviously – while having a look on recent or current armed conflicts in which democratic states are engaged – still an option for liberal democracies with regards to disputes with non-democratic states.
 Immanuel Kant, ‘Kant’s Political Writings’, Hans Reiss (ed.), H.B. Nisbet (trans.), Cambridge: Cambridge University Press, 1970, p. 114.
 Michael Doyle in his pioneering work, ‘Kant, Liberal Legacies, and Foreign Affairs’, Philosophy and Public Affairs, Vol. 12, No. 3 (Summer, 1983), pp. 205-235, applied the theory to what he called “Liberal states” which he defined as “States with some form of representative democracy, a market economy based on private property rights, and constitutional protections of civil and political rights” (207-208). The theory is sometimes called the “Liberal peace theory”. Within this paper, I use the terms “Liberal peace theory” and “Democratic peace theory” synonymously.
 For an excellent overview of the current ‘state of the art’, see James Lee Ray, ‘Does Democracy Cause Peace?’, Annual Review of Political Science, 1 (1998), pp. 27–46.
 Immanuel Kant, ‘Perpetual Peace’ (I795) in The Philosophy of Kant, ed. Carl J. Friedrich, New York: Modem Library, I949
 See e.g. Christopher Layne, ‘Kant or Cant: The Myth of the Democratic Peace’, International Security, 19:2 (Fall 1994), pp. 5–49; David E. Spiro, ‘The Insignificance of the Liberal Peace’, International Security, 19:2 (Fall 1994), pp. 50-86.
 David A. Lake, ‘Powerful Pacifists: Democratic States and War’, American Political Science Review, 86:1 (March 1992), p. 32. Democratic peace proponents often claim that democracies do not wage war against each other. Some scholars, inter alia David A. Lake, modified the thesis toward ‘less likely’.
 Jack S. Levy, ‘The Causes of War: A Review of Theories and Evidence’, in Philip E. Tetlock/Jo L. Husbands/Robert Jervis/Paul C. Stern/Charles Tilly (eds.), ‘Behavior, Society, and Nuclear War’, Vol. 1, New York: Oxford University Press, 1989, p. 270.
 See e.g. Condoleeza Rice (former secretary of state),’The Promise of Democratic Peace’, Washington Post December 11, 2005, accessed at: http://www.washingtonpost.com/wp-dyn/content/article/2005/12/09/AR2005120901711.html (visited January 22, 2009)
 Concerning this argumentation, see e.g. David A. Lake, ‘Powerful Pacifists: Democratic States and War’, supra note 6.
 See Randolph M. Siverson, ‘Democracies and War Participation: In Defense of the Institutional
Constraints Argument’, European Journal of International Relations, 1 (December 1995), pp. 481–490.
 Compare e.g. Michael Mousseau, ‘Democracy and Compromise in Militarized Interstate Conflicts, 1816–1992’, Journal of Conflict Resolution, 42:2 (April 1998), pp. 210–230; William J. Dixon, ‘Democracy and the Peaceful Settlement of International Conflict’, American Political Science Review, 88:1 (March 1994), pp. 14–32.
 Randall Schweller, ‘Domestic Structure and Preventive War: Are Democracies More Pacific?’, World Politics, 44:2 (January 1992), pp. 235–269. With the emerge of the Bush Doctrine of “acting against emerging threats before they are fully formed” within the National Security Strategy of the United States of 2002 and with the American intervention in Iraq, this claim of democratic peace is obviously falsified. See Duncan E. J. Currie, ‘”Preventive War” and International Law after Iraq’, accessed at: http://www.globelaw.com/Iraq/Preventive_war_after_iraq.htm (visited January 22, 2009).
 Zeev Maoz/Bruce M. Russett, ‘Normative and Structural Causes of Democratic Peace, 1946–1986’, American Political Science Review, 87:3 (September 1993), pp. 624–638.