This study offers a new interpretation of the theoretical basis of the political alliance and rupture between Alexander Hamilton and James Madison. The central thesis is that Madison was correct that his and Hamilton's disagreement was rooted in their different orientations toward republican versus monarchical governments. Although for the past century scholars have rejected Madison's claim that Hamilton harbored monarchical principles and intentions, this study argues that the textual record suggests that he did. More specifically, it is demonstrated that there is no evidence that Hamilton had a genuine principled commitment to republican government. Moreover, the evidence does indicate that he always believed America would be better served by emulating the British mixed regime, complete with a hereditary monarch, and that he sought to put the United States on a developmental path toward such a regime. This difference between Hamilton and Madison was based on both disparate political principles and differences in their prudential judgments about the possibility that the Americans could overcome what this study calls the "18th century critique of republics." This powerful tenet of Enlightenment political science claimed that two sociopolitical processes tended to transform republics into despotic or, at best, limited monarchical regimes. One of these processes, "the republican violent death," was thought to naturally lead republics into anarchy and eventually monarchy or despotism. The other process, "the republican security dilemma," consisted of several pressures placed on regimes by their external security environment to adopt policies and establish institutions that undermined the domestic requisites for republican liberty. The most salient implication of the 18th century critique of republics was that the British balanced constitution presented the best model for durable liberty under modern conditions. This study argues that Madison and Hamilton were united in taking this critique seriously and that they both believed the two processes could have led to despotic regimes throughout North America if something had not been done to curb what they perceived as the excessive democracy and sovereign pretensions of the State governments. Their principal prudential difference was that Madison, unlike Hamilton, believed he had found republican cures for these republican diseases.