The US and the Soviet Union were only allies because they had an enemy in common - Nazi Germany. The US, France and Britain sent military expeditions to Russia in 1918-1919 to try and squash the Russian Revolution that became the Soviet Union, they were never friends with the USSR.
Everywhere the US and British armies entered a country, they liberated it. Everywhere the Russian army entered a country, they forcibly put in a government controlled by Russia and made it a satellite of the growing Russian empire. Remember where the communist governments of North Korea and North Vietnam came from? Remember what happened to the Baltic Republics?
The US and British saw that the Soviet Union was literally trying to take over the world, and that someone had to push back or the Soviet-style government - which had already killed more than 10 MILLION of its own people before WWII even started - would be imposed on billions of innocent people. The US had the most resources, so it took on the lion's share of the defense. Things quickly got so tense that the US and its allies felt forced to do some bitter things to try and hold back their enemies - like support anti-communist governments that treated their own citizens as enemies to be brutalized.
The Cuban Missile Crisis is seen by most as the closest the US and USSR ever came to open war. US and Soviet pilots regulary fought each other during both Korea and Vietnam, but a diplomatic fiction that the Soviet pilots were actually Korean or Vietnamese was in place to keep things from escalating beyond those proxy wars.
