This chapter offers an introduction to early modern Just War theory by analysing one of the most important and influential treatises written on the subject: On the Law of War and Peace (LWP), by the Dutch philosopher and statesman Hugo Grotius (1583–1645). 1 First published in 1625, the book was tremendously popular for more than two centuries. It is infrequently read and taught today. Its massive size–a popular modern edition runs to almost 2000 pages–and its arcane style, full of legal casuistry and obscure references to ancient poets, puts offmany a well-intended reader. As does the bewildering range of topics covered in the book: LWP contains insightful analyses of property, sovereignty, and the nature of morality but also lengthy discussions of alluvial sediments and archaic burial rituals. Not exactly the kinds of themes one expects to find in a book on war and peace. 2 Sections more directly pertaining to war have equally befuddled modern readers. Oftentimes Grotius seems to permit conduct he elsewhere roundly rejects as unjust. This has allowed commentators to present deeply conflicting interpretations of Grotius. Some interpreters place him in the bellicose humanist tradition of Alberico Gentili (1552–1608) and Thomas Hobbes (1588–1679). 3 Others have argued that the Dutch philosopher is committed to “contingent pacifism”: while waging war is morally permissible under certain conditions, these conditions are almost impossible to meet in practice. 4 As this chapter reveals, LWP is in fact a highly systematic and original exploration of four sets of rules governing war: justice, morality, the voluntary law of