<human>:  Why did the Korean War start?
<bot>: The Korean War started in 1950 when North Korea invaded South Korea. The United Nations Security Council passed a resolution that called for an immediate ceasefire, but North Korea ignored the ceasefire and continued their invasion. The United Nations responded by sending troops to South Korea to help the South Koreans defend themselves. The Korean War ended in 1953, when the United Nations forces forced the North Korean troops to retreat back to North Korea.