This chapter focuses on NATO, the institutional core of Pax Transatlantica. Since the end of the Cold War, NATO has enlarged its membership and ventured beyond its immediate neighborhood. Its significance as a security actor has been enhanced, not least because of the actions of the Russian Federation in Georgia, Ukraine, and Syria. By 2020, NATO was bigger and more engaged than ever before, with military capacities that dwarfed those of any of its real or potential adversaries. Yet, the success story was hampered by widespread pessimism about NATO’s future on both sides of the Atlantic. In fact, the post–Cold War era had seen numerous inter-alliance crises: the Iraq War of 2003 being the most obvious example. Nevertheless, three decades after the collapse of the Berlin Wall, NATO retained its function as the basic building block of Pax Transatlantica.