The Cold War between the superpowers of the United States and the Soviet Union was a clash of distant ideologies in a changing world. Friction developed between the two on many occasions as either side tried to expand their spheres of influence in politics, geographical surroundings, and even space. Continued clashes between the US and Soviet Union began to tense their relations during this era as it became evident to all that the cord of discontent could snap at any given moment. Western countries wanted the liberated states of Eastern Europe to be ruled with a democratic government and a capitalist economy. Joseph Stalin felt entitled to rule the Eastern European countries it occupied in World War II. Stalin wanted these countries to be used as a buffer to protect USSR boarders. Communist governments in these Eastern European countries would be under the control of the USSR, thereby protecting it. The Russian army, ready to attack Berlin, was halted to that the Yalta Conference between the "Big Three- (Joseph Stalin, Winston Churchill, and Franklin D. Roosevelt) could determine the war's end. The purpose of the Yalta Conference was to re-establish the countries destroyed and conquered by Germany. Poland and Yugoslavia were given their own governments; Germany was to be divided into zones to be controlled by each of the four countries (US, Great Britain, France, and USSR). Berlin was promised to the USSR as part of the eastern half of Germany upon its surrender, and Russia agreed to join the war against Japan when the war in Europe was over. Communist governments were established and democratic supporters were suppressed. The US, still fighting Japan, did nothing to stop Stalin. The bombing of Hiroshima and Nagasaki not only ended the war, but also demonstrated the superiority of the United States over Russia. This demonstration of superiority established fear in the Russian government and pushed the Soviet government towards nuclear technology and militarization.