Why was Korea divided into two countries?

Korea was a Japanese colony prior to World War II. With the Red Army attacking in Manchuria and heading towards Korea, it was determined between the USSR and the U.S.A that Korea would be temporarily split by the two powers before it would later be united as an independent nation. A U.S. Army staff officer actually decided to use the 38th Parallel as the line to split the two occupying powers.

The Soviet Union wanted the country to be united together under communist rule. They inducted their leader Kim Il Sung as their chosen Communist leader. They at first expected that guerrilla warfare would help overthrow the southern government and create a Communist government.