For almost 45 years from 1945 to 1989 the worlds two most powerful countries opposed each other in what they call the "cold war". Fundamentally the cold war was a conflict of ideological and political differences between the United States of America (USA) and the Soviet Union (USSR). The struggle was called the Cold War because it did not actually lead to fighting, or a "hot"war.
The tensions between the USSR and the USA originated well before the end of the Second World War. It had all begun in 1917 when the Russian Revolution took place and Soviet Communism was born. The USA alongside its allies sent troops to try and destroy the Bolshevik Revolution of 1917 and try to crush communism altogether. The attempt had failed, and from then on neither side trusted each other again.
The hostility and rivalry between the USA and the USSR was put on hold in 1941 where both the countries alongside Britain became allies to defeat Nazi Germany. But there still laid a great mistrust between the two allies. Towards the end of the war the allied leaders began to have meetings to discuss Germany's future. The most important of these meetings was the Yalta conference in Crimea, in 1945. This meeting of the "Big Three" (Roosevelt-USA, Churchill-Great Britain, and Stalin (USSR) where the issues of a Liberated Europe, how Germany was to be divided, The issue about Poland, and what would happen in Japan. Although the leaders were working together in post-world war 2 conditions there still laid a great mistrust between them. It is suggested that at the heart of their disagreements was the way that the countries should be run. The USSR were communist, and The Americans were Capitalists. Both these countries felt threatened from one another.
One big major turning point in how the cold war unfolded was the Korean war. It was basically Communism Vs Capitalism. From the year 1910 to 1945 Korea had been controlled by the Japanese.