On June 27, 1950, North Korean forces, backed by the Soviet Union, launched a surprise invasion into South Korea, marking the beginning of the Korean War. The United Nations, led by the U.S., quickly intervened in support of South Korea. The war would last for three years, causing millions of casualties and leaving the Korean Peninsula divided. The conflict became a Cold War battleground, as the U.S. and the Soviet Union supported opposing sides. The war ended in 1953 with an armistice, but no official peace treaty was signed, leaving North and South Korea still technically at war.