This chapter examines how global terrorism, and particularly the war on terror, has shaped US foreign policy. It first provides an overview of the 9/11 terror attacks and definitions of terrorism before discussing the US experience of terrorism prior to 9/11 as well as the political environment in Washington at the time of the attacks. It then considers the response of the Bush administration in Afghanistan and Iraq as well as the nature and aims of the al-Qaeda organization. It also reviews the conduct of the war on terror in its first nine years, along with the decline and transformation of al-Qaeda after 2010. Finally, it analyzes the options available to the United States in the war against al-Qaeda, ISIS, and like-minded groups.