This chapter explores the consequences of war for water security and insecurity. It maps out and analyses four main ways in which war matters for water: through infrastructure destruction; through population displacement; through the expropriation of resources and infrastructures; and through war’s profound if mostly indirect ramifications for state-building and development. Empirically, the chapter draws on evidence from across the divided environments considered in this book, including the ongoing wars in South Sudan, Syria and Lake Chad, the 2003–5 Darfur war, recent Israeli wars on Gaza and key historical conflagrations such as the 1948–9 Arab-Israeli war. The chapter argues through all of this that war is deeply contradictory, being simultaneously highly destructive and highly productive in its water security consequences. And it argues that this is likely to remain the case in an era of climate disruption: while, for some, war is likely is have sharply negative climate vulnerability consequences, it is nonetheless also the case, the chapter shows, that adaptive capacities are often founded on infrastructures and hierarchies of political violence.