Great Britain, the United States, and the Soviet Union were the key actors in the Allied coalition. China, France, and other nations played crucial but smaller roles. The leaders who became known as the Big Three — President Franklin Roosevelt, Prime Minister Winston Churchill of Great Britain, and Premier Joseph Stalin of the Soviet Union — set military strategy. However, Stalin was not a party to the Atlantic Charter, which Churchill and Roosevelt had signed in August 1941, and disagreed fundamentally with some of its precepts, such as a capitalist-run international trading system. The Allies also disagreed about military strategy and timing. The Big Three made defeating Germany (rather than Japan) the top military priority, but they differed over how best to do it. In 1941, a massive German force had invaded the Soviet Union and reached the outskirts of Leningrad, Moscow, and Stalingrad before being halted in early 1942 by hard-pressed Russian troops. To relieve pressure on the Soviet army, Stalin wanted the British and Americans to open a second front with a major invasion of Germany through France.

Roosevelt informally assured Stalin that the Allies would comply in 1942, but Churchill opposed an early invasion, and American war production was not yet sufficient to support it. For eighteen months, Stalin’s pleas went unanswered, and the Soviet Union bore the brunt of the fighting; in the 1943 Battle of Kursk alone, the Soviet army suffered 860,000 casualties, several times what the Allies would suffer for the first two months of the European campaign after D-Day. Then, at a conference of the Big Three in Tehran, Iran, in November 1943, Churchill and Roosevelt agreed to open a second front in France within six months in return for Stalin’s promise to join the fight against Japan. Both sides adhered to this agreement, but the long delay angered Stalin, who became increasingly suspicious of American and British intentions.