During the dark days of 1940, through the Battle of Britain and the Blitz when Britain stood alone against the Axis Powers, he urged his compatriots to conduct themselves so that, "if the British Empire and its Commonwealth last for a thousand years, men will still say: 'This was their finest hour.'" He successfully resisted pressure from inside the War Cabinet for a compromise peace with Germany in May 1940 and placed his hopes for eventual victory on the intervention of the United States in the war on Britain's side. There was little sign of this during the summer of 1940, but with the successful outcome of the Battle of Britain, President Franklin D. Roosevelt decided to support Britain, not by direct American intervention but with naval assistance and military Lend-Lease aid. When Germany invaded the Soviet Union in June 1941, Churchill welcomed this new adherent to the Allied cause, this despite his implacable hostility towards the Soviet regime in the 1920s. He was overjoyed when the United States entered the war in December 1941 after the Japanese attack on Pearl Harbor. Churchill established close ties with Roosevelt and the Soviet leader Joseph Stalin, forming a triumvirate at the head of what he termed the "Grand Alliance". Travelling ceaselessly, he laboured to coordinate military strategy against Adolf Hitler and the Axis.

For a time Roosevelt generally adopted Churchill's strategic ideas, such as the prime minister's insistence on the invasion of North Africa in 1942 instead of a cross-Channel assault, which the American army chiefs wanted. However after 1943, as the United States had become immeasurably more powerful, Churchill was forced increasingly to accept American-imposed war plans, despite his vigorous courting of Roosevelt by means of frequent face-to-face meetings in the United States, Canada, and North Africa. Churchill's warnings after the Yalta Conference in early 1945 about Stalin's European ambitions were ignored-Roosevelt wanted to work with Stalin for a peaceful post-war order. British general elections were held during the Potsdam Conference, the last great "Big Three" conference in the summer of 1945, with Churchill present for part of the time. Given his popularity as wartime leader, he was not expected to be defeated at the election. However, the Labour Party won by a landslide. British public opinion was alienated by Churchill's repugnance for social and economic reform (he had taken very little interest in domestic policies during the war), nor did it wish to return to the slump and unemployment of the 1930s with which the Conservatives were now identified.

Post-War Career

Inevitably Churchill was critical of the "welfare state" reforms of his successor, Clement Attlee. He voiced his suspicions of the Soviet danger in his famous Iron Curtain speech in Fulton, Missouri, in 1946.