The Korean War began in 1950 when North Korea invaded South Korea, sparking a conflict that lasted until 1953. It involved military forces from 22 countries aligned with the UN defending South Korea against North Korea and China. Over its three years, the war progressed through six stages as forces battled back and forth across the Korean Peninsula, ultimately resulting in a stalemate and armistice that divided Korea along the original border. The war caused over 2 million military and civilian casualties between both sides.