The democratic peace proposition has been lurking in Western thought for millennia, as Weart 1998 shows, but Kant 1991 provides its first modern formulation. The idea that global democracy would provide a solid foundation for global peace was restated in 1917 by Woodrow Wilson as a justification for American entry into World War I and then as part of his vision for a new world order. Modern political science first observed the dyadic democratic peace—that democracies tend not to fight each other—in the 1970s. The observation enjoyed greater attention in the 1980s in particular in two pathbreaking 1983 essays by Michael Doyle, reprinted in Doyle 2011 . It received fuller theoretical and empirical attention in the 1990s. Fukuyama 1992 , a famous argument that humanity had reached “the end of history,” incorporates the democratic peace proposition. Other scholars sought to develop the theory and push forward more advanced research designs in works such as Russett 1993 ; Ray 1995 ; and Rousseau, et al. 1996 . In the 2000s, proponents of the democratic peace responded to their critics and embedded the democratic peace in a broader Kantian peace ( Russett and Oneal 2001 ).
Although some writers have questioned or even rejected the peace by trade proposition, their criticisms are not convincing. Beck, Katz, and Tucker (1998) raised the serious technical issue of time dependence in the time-series cross-section data, but Russett and Oneal (2001; see also Oneal 2003 and Oneal and Russett 2003b) responded to the objections raised against their earlier work and demonstrated that those objections do not affect their substantive conclusions. For a while, Hegres (2000) study seemed to necessitate a qualification of the peace by trade proposition. He found that the pacifying effect of trade is stronger among developed countries than among less-developed countries. More recently, however, Mousseau, Hegre, and Oneal corrected this earlier finding and reported: Whereas economically important trade has important pacifying benefits for all dyads, the conflict-reducing effect of democracy is conditional on states economic development (2003, 300). Gelpi and Grieco (2003) suggested another qualification. In their view, trade no longer pacifies relations between autocratic states. According to Mansfield and Pevehouse (2003), another modification of the peace by trade proposition might be required. The institutional setting, such as preferential trade agreements, matters. It is even conceivable that other forms of economic interdependence, such as cross-border investments, exercise some pacifying impact. Foreign direct investment (FDI) certainly promotes prosperity, growth, and democracy (de Soysa and Oneal 1999; de Soysa 2003), but the conceivable pacifying impact of FDI still lacks sufficient empirical investigation.
Instances of democracy that have been cited include Gopala in Bengal, the Polish-Lithuanian Commonwealth , the Althing in Iceland , certain medieval Italian city-states (such as Venice ), the tuatha system in early medieval Ireland , the Veche in Slavic countries, Scandinavian Things, and the autonomous merchant city of Sakai in sixteenth century Japan . However, few of these have an unbroken history into the modern period—an exception being the Althing, which lays claim to being the oldest parliament in the world. Furthermore participation in many of these post-feudal governments was often restricted to the aristocracy.