North Korea
Japan tried to suppress Korean traditions and culture and ran the economy primarily for its own benefit. Korean resistance groups known as Dongnipgun (Liberation Army) operated along the Sino-Korean border, fighting guerrilla warfare against Japanese forces. Some of them took part in allied action in China and parts of South East Asia. One of the guerrilla leaders was the communist Kim Il-sung, who later became the first leader of North Korea.
Division of Korea
With the onset of the Cold War, negotiations between the United States and the Soviet Union failed to lead to an independent and unified Korea. In 1948, UN-supervised elections were held in the US-occupied south only. Syngman Rhee won the election while Kim Il-sung was appointed as the leader of North Korea. This led to the establishment of the Republic of Korea in South Korea, which was promptly followed by the establishment of the Democratic People's Republic of Korea in North Korea. The United States supported the South, the Soviet Union supported the North, and each government claimed sovereignty over the whole Korean peninsula.