The Korean War began in 1950 when North Korea invaded South Korea, leading the UN to authorize military intervention led by the US to defend South Korea. The war became a proxy conflict between the US and USSR in their emerging bipolar world. Fighting involved intense ground combat, air battles between jet fighters, and use of new technologies like helicopters. After years of stalemate, an armistice was signed in 1953, though a peace treaty was never achieved, leaving the two Koreas still divided and technically at war.