After WWII, the U.S. and USSR emerged as rival superpowers. Ideological conflict: capitalism/democracy vs communism/authoritarianism. The conflict never became "hot" directly between the two, due to nuclear deterrence (MAD — Mutually Assured Destruction).
Containment: George Kennan's 1947 "Long Telegram" — the U.S. should NOT try to destroy communism, but CONTAIN its spread. This became THE guiding strategy for 40+ years of foreign policy.
AP Exam Focus (KC 8.1.I): Period 8 heavily tests: (1) Containment as the CENTRAL strategy, (2) Truman Doctrine vs Marshall Plan (military vs economic), (3) Korean War as "limited war" precedent, (4) McCarthyism's impact on civil liberties, (5) Cuban Missile Crisis as closest to nuclear war, (6) Détente as pragmatic diplomacy, (7) How Cold War shaped DOMESTIC policy (GI Bill, Highway Act, NASA).