Japan had to give up ruling Korea after World War II. Korea had been under Japanese colonial rule from 1910 until Japan surrendered in 1945.
Japan’s control began with the Japan–Korea Annexation Treaty of 1910, after years of increasing Japanese political and military influence. The colonial government, headquartered in Seoul, imposed Japanese administration and pursued assimilation policies, including restrictions on Korean language and culture.
Japan’s surrender on August 15, 1945, ended its rule over Korea. The peninsula was then divided into Soviet and American occupation zones near the 38th parallel, a division that later hardened into North Korea and South Korea.
A common mix-up is saying Korea became immediately unified and fully independent after the war. Although Japanese colonial rule ended, competing Allied occupations and Cold War politics created the division that led to the establishment of two Korean states in 1948.