Background to Korea
Korea became a Japanese territory in 1910. It had been a protectorate of Japan under the Japan-Korea Treaty of 1905 (Eulsa Treaty). However, the origins of Japanese interest in the Korean peninsula go back a bit further. The Japan-Korea Treaty of 1876 (Treaty of Ganghwa) forced the Koreans to grant extraterritorial rights to Japan. In other words, when in Korea, Japanese citizens were above the law. The First Sino-Japanese War from 1894-1895 had been over control of Korea. The Russo-Japanese War from 1904-1905 was over competing interests over Manchuria (Qing Empire) and Korea.
In the late 1800s-early 1900s, European powers were gaining power in the form of colonies and territories. Naturally Japan wanted to join this world power clique. It had proved its prowess when it defeated the Russian Empire in the Russo-Japanese War. In addition, Japan had been modernizing quickly under the Meiji. With this astronomical rise in power, Japan may have felt the need to protect its "Asian little brothers" from the Western imperialist powers. Of course this may have been a ploy on Japan's part to entrench themselves even more in neighboring regions. Japan very well could have been emulating its own oppressors. After experiencing US gunboat diplomacy (a gratuitous display of military power with hopes that that will sway foreign affairs) à la Commodore Matthew Perry, Japan used it against Korea for the 1876 treaty.
In the late 1800s-early 1900s, European powers were gaining power in the form of colonies and territories. Naturally Japan wanted to join this world power clique. It had proved its prowess when it defeated the Russian Empire in the Russo-Japanese War. In addition, Japan had been modernizing quickly under the Meiji. With this astronomical rise in power, Japan may have felt the need to protect its "Asian little brothers" from the Western imperialist powers. Of course this may have been a ploy on Japan's part to entrench themselves even more in neighboring regions. Japan very well could have been emulating its own oppressors. After experiencing US gunboat diplomacy (a gratuitous display of military power with hopes that that will sway foreign affairs) à la Commodore Matthew Perry, Japan used it against Korea for the 1876 treaty.
After the Japanese loss in World War II, the international community was at a loss as to what to do with Korea. It was decided that the peninsula be split into two--Communist trusteeship north of the 38th Parallel and Western/Capitalist trusteeship south of the Parallel. This isn't the only consequence of Japanese imperialism that has an effect on current affairs. Horrible accounts of comfort women and forced labor or conscription continue to spring up in the news. Island disputes also make for recurring stories in the news.