Why had the wartime alliance between the USA and the Soviet Union broken down by the end of 1946?
There are two main reasons for this.
First, the wartime alliance was based not on mutual trust but on mutual need. The Soviets and the Americans never really felt much of a bond. Each side was very suspicious of the other’s political and economic ideology. They were only allies because they both wanted Hitler and the Nazis defeated.
Second, the events that occurred late in World War II and in the months afterwards made each side more suspicious of the other. The Soviets seemed to be trying to expand into places like Greece and Turkey. They were not allowing free elections in Eastern Europe. At the same time, the US and Great Britain were producing rhetoric like George Kennan’s “Long Telegram” and Winston Churchill’s “Iron Curtain” speech.
Since the alliance was a shaky one to begin with, it was easy for events like those of 1946 to break it down completely.