There were many aspects to the Alliance breaking down between 1944 and 1945 these are the long term hostility between the USA and the USSR, the arguments at the Yalta and Potsdam conference and the different personalities in the big and the change of leadership within USA and Britain.
In 1917 civil war broke out in the USSR. The USA, Britain, France and the other countries decided to try and wipe out communism by force. They invaded the USSR in support of the White Russians, however although the White Russians were supported by the non communist countries the Bolsheviks managed to establish themselves. They renamed the country the Soviet Union and it became a Communist dictatorship which lasted till the early 1990's.
When it came to conferences like Yalta and Potsdam there was already hostility and tension between the big three because in previous years the USA and Britain had sent troops to try and destroy the very government they were trying to negotiate with.
The USA and the USSR had hugely different ideological differences which lead to further tension between the two nations. Stalin believed that all property was owned by the state on behalf of the people and that people were provided with the pay that they would need to live, on the other hand Roosevelt believed that land was owned by individual people and they were free to earn as much money as they wished. This meant that when Stalin and Roosevelt met at conferences like Yalta they had completely different principals about how their country should be run. This lead to further strain to their political relationship.
At Yalta the Big Three managed to make some agreements on the future of Europe and the world. Some of these agreements are as countries were liberated by the allies they would be allowed to hold free elections to choose their government. They also agreed that the USSR would help the USA defeat Japan.The only major disagreement was about Poland. The USSR wanted to move its boarders westward into Poland and in turn Poland could move its boarders into Germany. Roosevelt was not happy with this proposition but he could do little to refuse because the USSr's Red Army occupied the whole of Poland and the Eastern part of Germany. Britain and the USA finally accepted this on the terms that Stalin would not interfere with their plans in trying to stop communism spreading in Greece.
At Potsdam there were also agreements and disagreements. The Big Tree agreed that the Nazi party should be stamped out all over Germany. At Potsdam Stalin wanted the USSR to share in the occupation of Japan but Truman firmly blocked the idea. The USA and Britain also wanted to have a bigger say in what went on in Eastern Europe but Stalin also blocked this idea. On the other hand they did agree upon the final boarders of Poland although the USA and Britain disagreed. They also agree upon the reparations bill.
The various disagreements the leaders had whenever they met was primarily to do with their very opposing attitudes. The primarily hates were between Stalin and Churchill, who were sworn enemies, while Stalin and Roosevelt were far more amiable when they met. This was mostly due to their ideological differences based on their respective governments.