Contrary to the way Hobbes has been interpreted for centuries, the author argues that Hobbes laid the groundwork for contemporary international law and for a distinctly moral approach to the rules of war. The paper has the following structure. First, the author explains the role that the laws of nature play in Hobbes's understanding of the state of war. Second, he explains Hobbes's views of self-preservation and inflicting cruelty. Third, he reconstructs Hobbes's important insight that rationality governs all human affairs, even those concerning war. Fourth, he explicates the idea of cruelty moving from what Hobbes says to a plausible Hobbesian position. Fifth, he addresses recent philosophical writing on how best to understand the rules of war. Sixth, he then turns to legal discussions of cruelty's place in debates about the laws of war, showing how his Hobbesian approach can ground these laws.