This volume’s principal goal has been to de-provincialize the study of US foreign relations by exploring how people, capital, things, and knowledge have moved across borders since 1945. This chapter shows how US military members, materiel, and ideas all crossed borders in the first two decades of the twenty-first century, because of – or justified by – national security. The terrorist attack of September 11, 2001 produced a broad militarization of US foreign policy through three different presidential administrations – a pattern that was both a continuation and a departure from previous trends in American foreign relations history. Partly as a result of US actions, what followed were seven wars in Afghanistan, Pakistan, Iraq, Syria, Yemen, Somalia, and Libya – none of which have been fully successful.