The past role of Germany in European affairs meant that its future status was hotly contested. The American, British and French sectors were coalescing into a single entity, while East Germany was already following a Stalinist path. The difficulty was Berlin, also divided among the Allies, but stuck in the heart of East Germany. Free movement was possible across the city divide, and the Allies enjoyed rights of access to the city by train and air. Stalin found this Western outpost a provocation. In June 1948 he attempted to lay siege to West Berlin, and its 2.5 million people, by blocking all rail, road and canal traffic. The United States, Britain and France employed all available aircraft to lift vital supplies of fuel and food into Berlin to ensure that the people could survive. Eventually, in May 1949, Stalin lifted the siege. His ploy had backfired. Berliners began to be seen as doughty victims of communist pressure rather than just former enemies, and policy-makers were put on the alert, prompted to think about how they could prepare the Western world for a communist challenge that seemed to be becoming daily more dangerous.

A communist coup in Prague also provided intense symbolism: it was the failure to prevent German occupation of Czech territory that had exposed the folly of appeasement in the late 1930s. Barely a decade later Western policymakers did not want to make the same mistake again. Throughout the first decades of the Cold War the memory of Munich haunted Western leaders, urging them to stand up to aggressive dictators early lest they had to defeat them later on. The doctrine was resolute but it was not reckless. What Stalin had he would probably hold, but he must be allowed no more conquests. Soviet power could not be eliminated but could be contained.

Over time containment took the form of a series of alliances with potentially vulnerable states around the periphery of the Soviet bloc. The most important and durable – of these alliances was the North Atlantic Treaty Organization (NATO). Against the stormy political backdrop of the late 1940s, the United States committed itself to the future security of the European democracies. The instrument of this commitment was the North Atlantic Treaty, signed in April 1949, which contained the critical provision, in Article VI, that ‘an armed attack against one or more’ of the parties to the treaty would be ‘considered an attack against them all’. The comparable Soviet alliance – the Warsaw Pact – was not formed until 1955, ostensibly in response to West German rearmament. However, even by April 1949 a series of bilateral agreements between the Soviet Union and its satellites confirmed their readiness to follow Moscow wherever it might lead, including into war.

It took time before NATO became established as a body prepared to fight a European war under a supreme allied commander. Although American bombers had been moved to Britain during the Berlin airlift, the initial assumption was that Stalin had at this time no more stomach for another large-scale confrontation than did the Allies, and that the possibility of having to fight the United States would be sufficient deterrence in itself. It was only with Korea that the military dimension of the Cold War became fully developed.