Although there is wide agreement on the empirical fact of democratic peace today, there is fierce disagreement on its causes. Is it due to economic interdependence? Institutional restraint? Normative restraint? Perception of country similarities? American hegemony? What causes the contemporary democratic peace? All these factors undoubtedly have some explanatory power, but we argue that focusing exclusively on them misses a deeper cause of contemporary democratic peace, which is the heretofore undertheorized notion of "democratic solidarity" and mutual amity/support among democratic countries. Because democratic peace theory treats contemporary democratic peace as the mere absence of war (a "thin peace") and because of its rationalist character, it tends to neglect the "democratic solidarity" which underlies its ontological referent (democratic peace). Therefore it fails to recognize that today's "thick" democratic peace, built on democratic solidarity, is qualitatively different from the thin democratic/republican peace of past international systems. This failure has several consequences: a) it neglects the processes through which democratic solidarity and thick democratic peace reinforce each other, and; b) perpetuates an unawareness of the system-transforming potential of democratic solidarity. We support our arguments with a comparative case study, in which we show how different the thin peace among Italian city-states in the 1380-1454 period was from the contemporary, thick transatlantic peace. For example, the Italian thin peace was typically the result of historical accidents or factors exogenous to the arguments of democratic peace theory. Such a peace was based on coercion and/or cost-benefit calculations, was not stable or durable, and was unable to transform the Italian sub-system into a more peaceful one. On the other hand, the contemporary, thick democratic peace is based on a strong belief (war between democracies is unthinkable), is stable, enduring, and is showing the potential to push the "Lockean culture" of the international system towards a Kantian one. The structure of the paper is as follows. First, we analyze the different ways democratic peace literature and constructivist approaches conceptualize democratic peace, showing that while the first treats peace as a mere absence of war and ignores its reliance on democratic solidarity, the second can account for a thick peace built on common identity, interests, and values. Second, we produce a comparative historical analysis of the thick democratic peace of the contemporary international system and the thin democratic peace of the Renaissance Italian city-states. This analysis makes it clear that democratic peace theory must take into account democratic/republican solidarity issues and effects to fully understand the (re)production of democratic peace and its implications for the international system. ..PAT.-Unpublished Manuscript [ABSTRACT FROM AUTHOR]