As we all know, during WW2 the Soviets were allied with the United States, but after the end of WW2, was there any particular incident that broke apart the West's and Soviet's relationship and made them enemies, or was their "friendship", just a temporary thing until the bigger threat was taken care of?
The Cold War was announced before the end of WWII. Yalta Conference drew the new balance of power after the war with two strong blocks oposed both ideologically and economically. Each block pusued its own interests and that made them clash. This clashing didn't became a direct, open conflict due to the nuclear power that both blocks held. And, summarizing, that's all. It's not true that the US stood against tiranny, as the US has supported several tirannies that served its interests (I'm spanish and during most part of the Cold War we were under a tiranny that was directly supported by the US just because that tiranny was anti communist).
The US had capitalism and the USSR had communism, both countries hated the other's system.
The US had nuclear bombs and the USSR soon developed them, further straining their relationship.
Stalin was really evil and killed tons of his own people, the US was built upon standing against tyrants and being a democracy.
Both countries were remarkably wealthy and industrious with a lot of land and population. They both also had strong a military and were the two main super powers. The US had decided it kind of liked being the police force of the world after WW2 and both were vying for influence over the world. This created a natural rivalry.