After WWII, Eastern Europe was weakened and proved a prime target for a quick-fix government, which Stalin readily supplied from Russia. Western powers feared the spreading of Communism across Europe and saw that Greece and Turkey were barely holding onto their autonomy and that those two countries would be a catalyst for all other weak governments to fall to Communism under Stalin. On March 12, President Truman addressed Congress and asked for $400 million to militarily help Greece and Turkey from invasion. The president volunteered the Truman Doctrine as a piece of diplomacy that protected any country from Communism should it ask for help. The move of the west to help countries such as Greece and Turkey initiated the period of the Cold War with the USSR, who saw the political move as a seizure of power in a place whence they were not welcome.

The Cold War effectively changed America's foreign policy from isolationism to global interests. Once the Cold War began, the nuclear arms race began to heat up.

References

History of the Cold War