The Cold War began due to tensions between the US and USSR after WWII. The US was angry with Stalin for breaking promises made at Yalta and keeping the atomic bomb secret. The USSR wanted to spread communism while the US sought to contain it and give countries self-determination. This led to Europe being divided with an "Iron Curtain" between the Western democratic countries and Eastern bloc nations controlled by the Soviet Union. Germany was split into the capitalist West Germany and communist East Germany allied with the USSR.