How far was the Cold War, in the period 1945 to 1949, caused by Soviet expansionism in Eastern Europe?
Soviet expansionism began almost immediately after the defeat of Germany and the Western powers had no choice but to respond accordingly, thus escalating the tensions which became the Cold War. When the American, French and British zones of Germany were merged into the Federal Republic of Germany (then known as West Germany) the Soviet government claimed that this was a violation of previous agreements, and tried to take all of Berlin. The Soviets also refused to withdraw troops from Poland, Bulgaria and Romania, claiming they needed a "buffer" against Western aggression. This was a thinly veiled attempt to expand Soviet influence in the area, which was solidified by the founding of the Warsaw Pact. Later, the Soviets supported North Korean forces when they attempted to invade South Korea. In fact during the conflict, Soviet pilots flew a number of combat missions; but to disguise their involvement, the pilots wore Chinese military uniforms and spoke Chinese over their radios.
The Cold War was caused almost exclusively by this expansionism and the reaction of the US to it.
During the period after the end of WWII, it became increasingly clear that the Soviet Union was going to keep a firm hold on the countries of Eastern Europe. It had agreed to hold elections in those countries, but was clearly not going to do so. This (along with attempts by communist insurgencies to get control of countries like Greece, Turkey, and China) made the United States and other countries of the West very concerned. It was this expansionism, along with the concerns that it raised in the West, that caused the Cold War to begin.
It is also important to make light of the fact that both the US and the Soviet Union (USSR) were racing to spread their political ideologies (the US's ideas of democracy and the USSR's support of communism). Thus, both countries created economic plans that sought to help war torn countries rebuild after WWII. The US's plan was called the Marshall Plan (1948) and it sought to provide economic aid to countries that were resisting communism and rebuilding to form a more democratic style of government. The USSR then established their own plan called the Molotov Plan, which sought to provide economic funds to countries and keep them under Soviet influence. Thus, the US sought to stop the Soviet expansion via the Marshall Plan, while the Soviets sought to extend their control through the Molotov Plan.