The Japanese regime was marked by political oppression, exploitation, and cultural assimilation. Teachers were not allowed to teach in the Korean language, or to teach anything regarding Korean history or culture, and all dissidents were brutally killed. This caused the Korean nationalist movement to splinter. In 1919, the Korean people demonstrated, inspired by Woodrow Wilson's talk of autonomous nationhood. Thousands were killed and tens of thousands imprisoned as a consequence. In 1925, the Korean Communist Party was formed (this was the first communist party in an Asian country), and a provisional Korean government was established in China. This provisional government was headed by Syngman Rhee. The Communist party staged demonstrations several times, but these invariably resulted in the massacre of thousands of Koreans by the Japanese in retaliation. Korean rebels were especially active in Manchuria, a province in northeastern China where the provisional Korean government was based. These rebels were led by Kim Il Sung. Despite resistance, Japan maintained a powerful and repressive presence in Korea until after World War II.
In 1941, Japan bombed Pearl Harbor and sparked the war in the Pacific. The provisional Korean government declared war on Japan, which meant little militarily, but served to rally the Korean people against their oppressors.
In 1943, Winston Churchill of the U.K., Franklin Delano Roosevelt of the U.S., and Chiang Kai Shek of China issued the "Cairo Declaration". This stated that Korea would become free and independent in due course.
In 1945, Japan surrendered to the U.S. and the U.S.S.R. The two countries agreed that the United States would accept the Japanese surrender below the 38th parallel, while Russia would accept it above. This resulted in a split Korea, but the original plan was that the two halves would be unified. Various political parties were formed in Korea, all clamoring for rule.