Carl Watts outlines the shift in British security policy and examines the role played by the Foreign Office during the post-War period. April 1999 marks the 50th anniversary of the signature of the North Atlantic Treaty, which came into effect in August 1949. The Cold War is over, but NATO remains an enduring feature of international politics, as the recent intervention in Kosovo has demonstrated. It is therefore timely to reflect on the process by which NATO came into existence and, in particular, to consider the role that the British Foreign Office played in the formation of the Organisation. It will be noted in this article that Britain’s commitment to post-War European security arrangements marked a significant departure from the pre-War strategy of ‘limited liability’.