Following 9/11 the United States faced a situation of exceptional insecurity. In that period the Bush administration argued that certain international norms did not apply to US conduct. Its argument was underpinned by the claim that the United States was in a state of armed conflict or war with a new kind of enemy. The purpose of this book is to examine whether this approach outlasted the moment of insecurity that gave rise to it. More than a decade on from those attacks, and following a change of administration, what influence do these arguments have on American policy? To answer this question it focuses on four areas of policy: the use of force and the prosecution, detention and interrogation of suspected terrorists. It demonstrates how the Bush policy programme was contested by liberals and realists from the outset. Any expectation that the war on terror would end following the election of President Obama has, however, proven unfounded. Obama consolidated the liberal pushback against aspects of the Bush programme but the US has continued to argue a state of armed conflict exists. The scope of the battlefield and the definition of the enemy has been a source of intense debate but the fact that the Guantanamo Bay detention facility remained open long after the President promised to close it is indicative of the underlying continuity. It is argued that this is driven in part by domestic politics and in part by an understanding of how the terrorist threat is evolving.