The war started when North Korea invaded South Korea on June 25, 1950. The U.S. responded by sending troops and supplies to help South Korea fight back against the invasion.
The Soviet Union supported North Korea and China supported the Chinese communists who were fighting for control over China’s mainland. The U.S. and its allies fought against both of these countries during World War II (1939-1945), so they had a lot of experience fighting against each other already.
The US had been supplying military equipment to South Korea since 1948 when it signed an agreement with the country’s government. But after North Korea invaded South Korea on June 25, 1950, even more, US aid was sent to help the South Koreans fight off their aggressors.
In September 1950, President Harry Truman ordered US ground forces into South Korea and General Douglas MacArthur was appointed Supreme Commander of UN forces there. The first major battle between American and North Korean forces occurred at Osan, where American soldiers were forced to retreat after facing fierce resistance from enemy troops.
The United States was worried that if North Korea took over South Korea, it would be able to launch attacks on Japan or even invade it someday. That would mean that Japan might have to depend on the United States for protection instead of relying on itself.
