The USSR fought alongside the USA and Britain to defeat Germany and Japan in WWII. However, by the end of 1945, relations were poor for several reasons:

In 1945 Truman became President of the USA and he was much tougher on the USSR than the previous president.
The USSR were angry that that the USA didn’t tell them before dropping atomic bombs on Japan and were worried by the USA’s nuclear weapons. In 1949 the Soviet Union developed their own atomic bombs.
After Germany was defeated, the USSR set up Communist governments in the countries of Eastern Europe (e.g. East Germany and Poland). This went against their earlier promises and it scared America that Russia wanted to take over more countries.
Winston Churchill made his famous Iron Curtain Speech which made Americans more worried about the spread of communism in Europe.
In 1947 President Truman announced his Truman Doctrine or Containment Policy. This was a promise to help any country anywhere in the world that was fighting against communism.
In 1948, the USA began its Marshall Aid programme which sent hundreds of millions of dollars to struggling countries in Western Europe. This was because they realised that poverty made countries more likely to support communism.

Finally, in 1949, China fell to communism, which made the US believe that more countries might do too. Stalin also blockaded West Berlin to try to force the Western powers out (although this failed).

During the 1950s, the USA's fear of communism intensified. This period was known as the Red Scare and was fuelled by Senator Joe McCarthy who led a witchhunt against suspected communists in America.

GCSE Conflict in Asia 1950 - 1975 AQA

Short Term Causes of the Cold War