This book assesses the unintended consequences of the proliferation of the laws of war. In 1856 there was one codified law of war. In 2018, there are over 70 such laws, and they place increasing constraints on belligerents. This increase has generated significant consequences for the commencement, conduct, and conclusion of both interstate and civil wars. States fighting interstate wars today prefer not to step over any bright lines where the laws of war would apply unequivocally. Thus, these states have stopped declaring war and concluding peace treaties. Rebel groups – particularly, secessionists that seek their own independent state – by contrast, have increasingly engaged with the laws of war. Secessionists are relatively unlikely to target civilians, and there is an increasing rate of peace treaty usage in civil wars that contrasts with the decline in interstate war. This research is based on two new major original datasets as well as a series of case studies, and is particularly unusual in combining analysis of interstate and civil wars.