The Soviet Union and the United States had always been uneasy allies, because they were ideological opposites. The United States was a capitalist power with a democratic government, and the Soviet Union under Stalin was a communist dictatorship. Their alliance broke down over disagreements about postwar Europe.
The Soviets had accepted Franklin Roosevelt's insistence that free elections be held in Poland, occupied at the end of the war by Soviet forces. After Roosevelt's death, with the war at an end, the Soviets did not allow these elections, installing instead a Stalinist-style regime. This caused fierce disagreement at the Potsdam Conference in the summer of 1945.
In August, the United States ended the war with Japan by dropping two atomic bombs, launching the atomic race that would be a reality throughout the Cold War. Other events accelerated tensions between the US and the Soviets. In the spring of that year former British Prime Minister Winston Churchill delivered his famous "Iron Curtain" speech in Missouri, claiming that the Soviet Union was attempting to bring all of Europe under communist control. His speech echoed one by Stalin earlier in the year—the Soviet dictator had stated that the United States and the Soviet Union could not be coexist.
Tensions were also increased as Greece and Turkey faced civil war between communist and anti-communist factions. So by the end of 1946, an atmosphere of mutual distrust and outright hostility prevailed between the United States and the Soviet Union.