The Cold War began as the Soviet Union occupied Eastern Europe toward the end of World War II. Roosevelt was going to push Stalin for free elections in the East after the war ended; however, Roosevelt died just before Germany's surrender. Stalin did not trust the West, thinking that the West wanted to keep the Soviet Union weak and isolated. Roosevelt's successor, Truman, did not trust the Soviets either. Truman did not share atomic secrets with the Soviets, even though the Soviets developed their own bomb six years after the United States used theirs on Japan in 1945.
During the early part of the Cold War, British Prime Minister Winston Churchill stated that an Iron Curtain had been drawn across Europe; this is where the term "Iron Curtain" originates, as it refers to countries under the control of the Soviet Union. George Kennan, from the US State Department, issued the Long Telegram, which stated that the Soviet Union was inherently evil and would stop at nothing in spreading its ideology all over the world. The solution to this was containment. When China fell to the communists under Mao, this communist spread appeared to be happening even though Mao had been fighting the Nationalist Chinese under Chiang Kai-Shek well before World War II. The United States organized the West into NATO. The Soviet Union organized Eastern Europe into the Warsaw Pact—once again, the world was balanced between two competing camps, as it had been before both world wars.
Stalin's successor, Khrushchev, was more open, but he was also willing to expand Soviet influence into the Western hemisphere; this led to the Cuban Missile Crisis, an event which brought the world to the brink of nuclear annihilation.
Both the United States and the Soviet Union developed their nuclear arsenals in a policy referred to as "mutually assured destruction." Since both sides did not want to risk nuclear war, they fought wars via proxies. The Soviets backed anti-imperialist groups in the developing world who agreed to take Soviet advisers, weaponry, and money in return for military alliances. The United States backed rightist leaders whose only positive attribute was a hatred of communism. These proxy wars existed off and on for over forty years in Korea, Vietnam, Afghanistan, and Latin America. In addition to proxy wars, both sides spent billions of dollars on new weaponry, propaganda, and espionage in order to try to gain an advantage.
The Cold War was built on the ruins of World War II. The United States and the Soviets were allies during this war, but it turned out to be an alliance of convenience, because the only thing the two sides agreed on was that the Nazis were evil. The Soviets and the United States accused each other of trying to take too much control of world affairs. The United States did not like the anti-capitalist and atheistic stance of the Soviets. The Soviets did not like the United States's meddling approach to world affairs. This set the stage for the Cold War. This tension shaped world affairs throughout the second half of the twentieth century and even explains some hostility between Russia and the West today.