The reason for the Cold War was this: the USA and the USSR had strong opposite views about government. Both believed that their government was the best way to run a country ans both were convinced that the other was trying to destroy their way of life.
The USA's government system was a democratic capitalist one. This allowed freedom of speech, different wages and there were huge differences between the rich and the poor.
The USSR on the other hand was communist and had a dictator. The state was in charge of everything and the people had little freedom of speech and there were lower standards of living.
The first origin for the Cold War was in 1918 in the Russian Revolution when the USA and other western countries supported the Tsar and his government and consequently they fought against the communist leaders that were later their allies. This fighting left a bitter taste in the mouths of Stalin and other communists but the USA were also also bitter as the monarch was killed and the communists came to power.
The two sides put aside their differences as they both fought against Hitler during World War Two. Although they supported each others efforts, there were disagreements behind the scenes to the USA's involvement. Stalin was angry that the American troop joined so late and he believed that because of the huge sacrifice of Russian soldier's lives, that gave the USSR benefits when it came to negotiating tat the end of the war.
In February 1945, the Big Three (Stalin, Churchill and Roosevelt) met at Yalta to discuss what would happen to Germany once the war had ended. It was agreed that Germany would be spit into four zones and Berlin, which was well within the Soviet zone, would also be split into four. There was also the question of what to do with the countries that the USSR had 'liberated'. The West wanted free and fair elections while Stalin wanted them to remain as a 'Soviet sphere of influence' to act as a buffer zone if the west should want to attack again. However he agreed to the elections.
In July 1945, the war had ended and the Big Three (now Stalin, Truman and Clement Attlee) met at Potsdam to carry through what they had discussed at Yalta. However, Stalin and Truman didn't get on as well as Roosevelt had and Truman was far more anti-communist. There was also tension because the USA hadn't come clean about their nuclear testing even though a Soviet plane had spotted the testing, they denied…