This chapter assesses the enduring relationship between the military role of the United States in Europe, through its participation in the North Atlantic Treaty Organization (NATO), and European integration from the Cold War to the present. It argues that, during the Cold War, western European security cooperation was conceived as part of a wider endeavour, which also included the United States and Canada. Also after the end of the East--West division, diverging priorities among the European countries and their preference for intergovernmental rather than supranational cooperation, together with US determination to preserve the transatlantic alliance, bolstered NATO’s role as the bedrock of European defence, while confining the role of European institutions to the range of peacekeeping and crisis management tasks. After reviewing the current state of the art of research on European security and defence, the chapter proceeds as follows. The first section focuses on the relationship between transatlantic and European security in the late 1940s, showing how western Europe’s security initiatives, such as the Dunkirk Treaty and the Brussels Treaty Organization (BTO), endeavoured to secure a US pledge against the Soviet threat rather than to foster defence integration in Europe. The second section debates the project of a European Defence Community (EDC) in the early 1950s, emphasising diverging west European perceptions of the EDC and of West German rearmament. More specifically, France viewed the EDC mostly as an intergovernmental toolbox to control the Federal Republic of Germany (FRG), rather than a truly supranational organisation. This section also argues that the British declined to participate, dreading the prospect of undermining NATO. After the EDC’s failure in 1954, the creation of the Western European Union (WEU) unequivocally left west European defence under the US umbrella.