The shift from ally to enemy
Paranoia struck deep after World War II. The Soviet Union had been torn apart by German troops and had lost millions of people fighting alone on the ground in Europe for two long years before Britain and the U.S. finally got around to the Italian and the D-day invasions.
The Soviet leadership saw the world as an anti-Communist conspiracy, and they were on a self-protective and ideological crusade to turn other countries Communist. They especially wanted a protective barrier of Communist satellite countries between themselves and Germany, a country that had torn into Russia twice in 25 years.
The Soviets knew that some in the West had put up with Hitler for years partly because they thought Hitler could kill off Communism. During the war (before he was vice president), Truman had said the U.S. should let the Germans and the Soviets kill each other off and help whichever side seemed to be losing to keep the bloody fight going. Having lost 27 million people (and almost having lost their country as well) in World War II, the Soviets had reasons to worry.
So did the United States. Not only did its so-called Soviet allies become increasingly hostile, but within a few years, the Soviets also developed the atomic bomb. Other countries, including China and Cuba, became Communist. To people in the U.S., it looked like a conspiracy was afoot in the world; how else could the Communists become so powerful?
The escalation of perceived threats
Like any argument with paranoia on both sides, push came to shove pretty quickly between the U.S. and the U.S.S.R. because each side was sure that it was right and that the other side was out to get them. The United States shut off Lend-Lease aid to the Soviets shortly after the end of the war. The U.S. offered to include the Communist countries of Eastern Europe in the Marshall
Plan to rebuild Europe. The Soviet leadership refused to participate and forced the satellite countries to do likewise, partly due to suspicion of American motives.
The Soviets clamped a lid on Eastern Europe and supported any country that said it was Communist. In response, the United States supported most countries that said they were anti-Communist. The showdown lead to a dangerous arms race, plus a standoff in jointly controlled Germany and shooting wars (officially police actions or conflicts) in Korea and Vietnam.