In 1866, the US sent a merchant steamer called General Sherman to Korea to negotiate a trading treaty, although some Koreans claimed that the real purpose was to plunder treasure from the royal tombs near Pyongyang, due to the boat being suspiciously heavily armed.
Although events are significantly disputed, it's generally thought that Korean forces attacked the boat because it passed into Pyongyang without permission while firing cannons.
The incident is often seen as an example of American interventionism and arrogance, and was even commemorated by a North Korean postage stamp in 2006.
In retaliation, the US mounted an expedition to Korea in 1871, known as the Shinmiyangyo, or the Korean Expedition - their first military action in Korea, but not their last.
American land and naval forces arrived on the island of Ganghwa, claiming to represent diplomatic delegations, trade relations, and to find out what happened to the General Sherman.
However, Korean policy dictated that foreign ships were not allowed to sail on the Han River, and so this expedition resulted in an armed conflict.
More than 200 Koreans were killed - but only three Americans. After the expedition, Korea became increasingly isolationist and wary of foreigners, refusing to negotiate with the US for more than a decade.
South / North Korea partition
The Allied victory in World War II ended the Japanese Empire's rule of Japan, and many Koreans demanded independence.