the North Korean army crossed the 38th parallel (created in 1945 to separate the Soviet-supported Democratic People’s Republic of Korea (today’s North Korea) and the U.S.-supported Republic of Korea to the South)
U.S. got involved
Invasion of innocent noncommunist in South Korea
start
The unwanted spread of communism by North Korea destroyed South Korean cities
The US got involved to contain communism and to stop the domino effect.
The North Korean allies consisted of China and the Soviet Union while the South Korean allies consisted of the United States and the United Nations.