This question--#1—basically asks you why the US and Great Britain ended up enemies with Stalin's Russia just a year or two after the conclusion of a very successful alliance against Hitler.
To answer this, you have to establish first what happened in l9l7. In that year, Woodrow Wilson took the United States into world affairs with his stated intention of “making the world safe for democracy.” Watching from across the sea, Wilson thought the cause of the dreadful European war was a lack of democracy—people not having a choice in what kind of government they lived in. Wilson thought that if people had input into where they lived and could choose their leaders in elections, there wouldn’t be any more wars.
In that same year, Vladimir Lenin and the Russian Communists took power in Russia and advanced their own utopian vision of the postwar world. They wanted to bring socialist revolution to the peoples of the world. They promised an end to rich and poor, a guaranteed standard of living, justice for those who had had no justice…in exchange for life in a dictatorship, in which a small group of men would make all the decisions for everybody.
These visions were diametrically opposed to one another. The Wilsonian vision required self-determination and elections; the Communist vision aimed to establish a kind of worldwide dictatorship—a dictatorship for the working classes, but a dictatorship nonetheless.
In the l920s and 30s, the conflict between the two nations was less acute, as each nation focused on its domestic priorities. There was even some improvement in the l930s, as the USSR launched a drive for industrialization and appeared to be in much better shape than the US and the capitalist world. Diplomatic relations were established between the two nations in l933. But as World War II approached, Stalin opted for a non-aggression pact with Hitler, which shocked and appalled the US and Britain. It looked like a cynical move designed to advance the Communist revolution, or at least gain a lot of territory.
In the second part, you need to demonstrate what changed. So connect these dots:
USSR invaded by Hitler
The only viable strategy to defeat Hitler: Make him fight on two fronts.
Decision by Great Britain(and later US)to offer the hand of friendship to Stalin
Then consider what had NOT changed in the relationship, connecting these dots, or some of them anyway:
What both nations wanted from the war : Atlantic Charter vs. Stalin’s insistence on control of territories.
The conflict over when to launch the invasion of Europe
Stalin and the USSR coming as liberators, also conquerors.
In the third section, you need to show what happened when Hitler was defeated. Again, connect these dots:
Yalta and Potsdam conferences: conflict over what should happen in E. Europe
Stalin’s reaction to the atom bomb
The dueling speeches of l946 and the demonizing of one side by the other, and vice-versa.
Now sum it up: doesn’t it seem reasonable to say that the two sides had only Hitler in common? The disappearance of Hitler meant the end of commonality between the two sides. It was back to business as usual, i.e. serious and sharp conflict in world views.