During the Reformation and the Age of Exploration, just war thinkers were forced to reexamine the premises on which the Augustinian tradition had stood, including their understanding of natural law, justice, and sovereignty. This chapter examines three thinkers crucial to that transition: Alberico Gentili, Francisco Suarez, and Hugo Grotius. They are part of the Augustinian tradition, but clearly show signs of subtle departure from their predecessors. Grotius, especially, is a hybrid between the Augustinian past and Westphalian future. They understood themselves to be engaged in a project of continuity: they wanted to salvage and reinterpret the intellectual inheritance of Christendom and reapply it to the changing and fracturing landscape of their day. But the new age inaugurated by the treaties of Westphalia transformed it in subtle but important ways, most prominently by secularizing its discourse and changing its understanding of natural law.