The wartime alliance between the US and USSR broke down after World War 2 for several key reasons: 1) The USSR lost around 20 million people in the war while the US/UK lost far fewer, making Stalin determined to increase Soviet security. 2) The US and USSR emerged from the war as the only true "superpowers" but had very different economic ideologies of capitalism vs communism. 3) At Yalta and Potsdam, the US and USSR disagreed over influence in liberated countries, with the USSR favoring communist groups in Eastern Europe over non-communist alternatives preferred by the US.