Explain the evolution of the cold war

Between World War I and World War II, the United States and the Soviet Union held a great deal of animosity towards each other. The US was capitalist, while the Soviets were communist, and they just didn't get along. When WWII hit, however, these two nations joined forces to defeat a common enemy: Hitler's Germany. After Germany's defeat, however, the US and Russia went back to being enemy's, although this time both sides have nuclear weapons. Both sides knew that to fire on the other with these weapons would likely lead to the destruction of humanity, so no shots were fired. Thus, the "Cold War"