The Cold War began between the USA and USSR after WWII. They had different economic systems (capitalism vs communism) and forms of government (democracy vs dictatorship) which caused tensions. At the Yalta Conference, Stalin resisted removing troops from Eastern Europe and installed communist puppet governments, drawing an "Iron Curtain." Both sides built nuclear weapons and used deterrence and containment policies to prevent direct conflict, instead fighting through proxies in conflicts like Korea and Vietnam. They formed opposing alliances like NATO and the Warsaw Pact.