Why did the korean war happen

The Korean War, occurring between June 25, 1950, and a cease-fire on July 27, 1953, was a war fought in Korea that was divided by the post-World War II Soviet and American occupation zones, with large-scale participation by other countries.

It was about communism; the united states did not want other nations to succumb to communism, the dommino effect basically. 

The answer is william henry harrison pursued and defeated british forces retreating into canada.