The book places international law at the centre of the transformation of United States (US) counterinsurgency (COIN) doctrine and practice that occurred during the Iraq and Afghanistan wars. It contradicts existing theoretical assumptions and claims international law matters much more than is often assumed and much more than scholars and practitioners have previously been able to claim. In particular, the book contends international law matters in a case that may be regarded as particularly tough for international law, that is, the development of a key military doctrine, the execution of that doctrine on the battlefield and the ultimate conduct of armed conflict. To do so, the book traces international law’s influence in the construction of modern US COIN doctrine and assesses how international law’s doctrinal influence has held up in Iraq and Afghanistan. My account of this doctrinal change is based on extensive access to the primary actors and materials. I argue we can trace international law’s impact on counterinsurgency via three pathways. The first incorporates international law’s ‘ideational influence’, primarily through the way in which a deference to the rule of law implicates specific rules of international law directly or indirectly. The second looks at international law’s influence on legitimacy, by which I mean the way international law is used to articulate and also to demonstrate legitimacy. The final pathway examines what I term international law’s mandatory impact, seen largely through its interaction with domestic law and domestic institutions. The book tells us something new about international law’s impact on the preponderant power in the international system, the US, in an area of state behaviour usually assumed to be inoculated from its influence. Nine years ago, in 2005, the US was accused of being a ‘lawless hegemon’, one that deliberately avoided rules of international law, especially those relating to the law of armed conflict (LOAC) and international human rights law.