The Cold War began between the United States and Soviet Union after World War 2. Tensions arose due to differing ideologies around democracy, capitalism, and communism. The US pursued a containment policy to stop the spread of communism through measures like the Truman Doctrine, Marshall Plan, NATO alliance, and military buildup. The outbreak of the Korean War in 1950 further escalated Cold War tensions and fears of communism grew domestically in the US through investigations led by McCarthyism.