In 1904, Korea was colonized by Russia and Japan as they were fighting over that region. With the Japan-Korea Annexation Treaty of 1910, Korea was forced to recognize they were subject to Japan law. The Japanese rule lasted until 1945, when America forced them to surrender. Korea was to build themselves back up from there. The next premier, Kim Il Sung helped to do this. From here started the Korean war.
China and North Korea share a border and China had been going through a decades long civil war. It was interrupted by WWII but continued after the end of WWII. The Chinese Nationalists lost to the PRC, resulting in their exile to Taiwan in 1949. The PRC was about to play a large role in Korea's future.
In June 1950, the North Korean communist government crossed the 38th parallel and invaded the democratic South Korea. Kim Il Sung, leader of the north, wanted to unify all of Korea into his communist regime. He had many soldiers and continued to move into the south. People of the south retreated and some even moved to the north.