Answer:
By the Cold War.
Step-by-step explanation:
After WWII, America and the Soviet Union knew they were BOTH international superpowers based on their success in defeating the Axis powers with nuclear weapons. Knowing this, the Soviets began expanding what I believe was south/east, and America felt threatened (somehow) to this process. So the non-violent "war" known as the Cold War (because the Soviet Union was cold and so is the USA) began.
I recommend the channel "Oversimplified" for further explanation.
(Before you ask, I do not sponsor "Oversimplified" or any channel for that matter.)