The spread of communism didn't stop in China. In 1925, Korea was occupied by Japan. At that same time, Karl Marx's book The Communist Manifesto had spread in Korea. A secret society of men met and established the Communist Party of Korea, based on Marx's ideologies. Belonging to a communist party was illegal under the Peace Preservation Law that Japan had imposed on the Koreans.

Kim Il-Sung, a Korean from the north, was one of the communist party members who fled to China where he joined Mao and the Chinese Communist Party. Later on, he would relocate to the Soviet Union. It would be there that Kim Il-Sung would become a staunch supporter of pro-Soviet communist thinking.

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Over the course of the next decade, Japan invaded China as well as continuing to occupy Korea. Both Chinese and Korean communists used guerilla warfare (the use of ambushes and surprise tactics) against the Japanese forces.

Once Japan had surrendered after World War II, Korea was divided into U.S. and Soviet occupation zones. The goal was to unify the country and let the people of Korea decide its destiny. Because the Soviet Union was a communist country at the time, it relied on exiled Communists who had fled to the Soviet Union to return to Korea to support pro-Soviet policies. Kim Il-Sung later returned with other Soviet-trained Koreans to establish a communist provisional government under Soviet support in what would become North Korea.

On June 25, 1950, armed forces from North Korea crossed the 38th Parallel (the border between North and South Korea) and invaded South Korea. The goal of the North Koreans under Kim Il-Sung was to unify Korea under a communist government. However, the South Koreans, backed by the United States, saw this as a threat to democracy. This was the start of the first significant armed conflict during the Cold War. (To review the Cold War, click here.)

Click below to listen to a audio segment (2:00) from the UN Security Council's decision to send UN troops into Korea to aid the South Koreans. Take notes as you listen.

Reaction to North Korean Surprise Attack

Why do you think the North Korean invasion of South Korea was deemed an "attack on the United Nations"?