Germany. Thirdly the reason for the breakdown is because of the Soviet aggression in Eastern Europe. America wanted to keep Eastern Europe contained but Stalin wanted to expand the Soviet Union. Finally I believe that because the East and West were now fighting Germany the
West never forgot the agreement that Stalin had made with Hitler in
1939 and feared that he might try and make a separate peace with
Germany.
The U.S decided that if the Soviet Union took over all of Eastern
Europe they would have the power to expand further, leaving America the only democratic Country in the world. To help prevent the spread of communism Truman created a policy to help contain Eastern Europe.
This was called the Truman Doctrine and was made from the consequence of Marshall. Marshall said that Communism spread easily in Countries were poverty was bad. At the time most of Eastern Europe was still suffering the affects of World War 1, so communism could spread easily if nothing was done. Marshall said that Eastern Europe would need $30 million to stop poverty and be contained using the Truman Doctrine.
This was known as Marshall aid. These policies together were the actions led by the Americans to contain Eastern Europe and not spread communism any further.
To a certain extent the USA were responsible for starting the Cold war for several reasons. Firstly Roosevalt and Truman never really had a sturdy relationship with Stalin. Secondly Truman was very anti-communist and distrusted Stalin, therefore disagreements were
frequent.