The Cold War began in 1945, as the United States and Soviet Union turned to face one another after defeating Hitler's Germany and Japan.
While fighting the war, each side portrayed the other as an ally again fascism. America provided substantial assistance to the Soviet Union, whose people and military bore a great deal of the fighting and dying. Washington and Moscow negotiated key agreements at Yalta and Potsdam to structure the post-war world. But as soon as Hitler was gone, the Soviets began breaking their promises. They established puppet regimes in Eastern Europe, alarming and angering the US and "free" European countries.
Suddenly the Soviet leader sometimes called "Uncle Joe" (Stalin) during the war came into focus again as a brutal dictator with an expansionist agenda.
How did the Cold War begin? Some argue it was incubating for years after the Bolshevik Revolution -- and World War II was a "time out" in an inevitable clash between the communist Soviet Union and capitalist United States. From this perspective, the origins of the Cold War are simple: World War II left two major powers standing, both with visions of spreading their incompatible ideological systems. To paraphrase an old saying, when there are only two tough guys on the block, there's a good chance of a fight.
Political and diplomatic disputes started even before the official end of the World War, including air and ground skirmishes. By 1950, US and Soviet pilots would be killing one another as part of the superpower proxy war in Korea, the first major armed conflict of the Cold War.