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Japanese annexation of Korea

After the annexation treaty in 1910, Japan annexed Korea and renamed the city Gyeongseong ("Kyongsong" in Korean and " Keijō " in Japanese). The city saw significant transformation under Japanese colonial rule. Imperial Japan removed the city walls, paved roads, and built Western-style buildings.

Seoul was deprived of its special status as the capital city and downsized under imperial Japan, compared to the traditional notion among people of the Joseon dynasty that Seoul included the area of approximately 4 km (2.5 mi) radius surrounding the Fortress Wall (i.e., Outer old Seoul; 성저십리; 城底十里). On October 1, 1910, Imperial Japan demoted Seoul as no different than any other city within the Gyeonggi province. After Imperial Japan's redistricting, Seoul only included the area inside the Fortress Wall and present-day Yongsan-gu. In the 1930s, as part of Imperial Japan's war efforts leading up to the Second Sino-Japanese War, Yeongdeungpo-gu was annexed into Seoul on April 1, 1936 to function as an industrial complex for steel and other metalworking factories.

The city was liberated by U.S. forces at the end of World War II.