Everyone knows that US troops, under the umbrella of the United Nations, entered Korea in June 1950 to counter the North Korean invasion. But most people don't know that this was not the first time US troops fought in Korea. The first US-Korean war was 79 years before, almost to the day.
In the 1850's, the Europeans and the Americans all began establishing spheres of influence in the Far East, establishing trade relations and, usually, installing compliant governments. After the "Opium Wars", China was divided up between the British and the Germans (with the US demanding an "Open Door" for trade of its own). In 1853 the US Navy sailed into Tokyo Bay and forced the Japanese Shogun to sign an agreement opening Japan to Western trade. By the time of the American Civil War, one of the last holdouts against Western encroachment was Korea, ruled by the Joseon Emperor, who, after seeing what had happened to China and Japan, was determined to keep the foreigners out of Korea and prevent a similar fate.
Korean court officials, 1871
The first attempt at contact between the West and the far-away and little-known country of Korea was in 1853, when the US Navy gunboat South America stopped at the port city of Pusan on the way to Japan. Although the Americans were treated politely by the Korean Court, the Emperor made it plain that he did not want relations with the West. Over the next 10 years a number of American ships were wrecked off the Korean coast, and the Korean government transferred the survivors to China, who then passed them back to the US.
In 1866, a British trading company sent a heavily-armed merchant ship, the General Sherman, to Korea to establish trade relations. The Joseon Emperor denied them permission to enter Korea, but the General Sherman sailed into the Taedong River anyway and approached the city of Pyongyang--then seized a Korean General who approached the ship and held him captive. In response, the Koreans set the General Sherman afire and killed her entire crew, including a number of Americans.
In 1871, the US Government decided to send an official delegation to Korea to seek a trade agreement, to negotiate an treaty about returning shipwreck survivors, and to determine what had happened to the Americans aboard the General Sherman. Although it was intended as a peaceful diplomatic mission, the fleet was heavily armed--it contained five Navy gunboats, twenty support ships, 600 Navy sailors and 100 US Marines. The fleet landed at present-day Inchon, near the Korean capital at Seoul (then known as Hanyang), in May. Korea at that time was being ruled by a Regent for the child Emperor. At first, things were polite; the Koreans treated the Americans well, but informed them that they were not interested in any trade agreement. On June 1, the Americans told the Koreans that they were going to send a few ships to explore the area. But when two US gunboats sailed past Ganghwa Island in the Han River, a forbidden area near the capital, the Korean shore batteries there opened fire on them. Most of the shots missed, and nobody was injured, but the US commander, Rear Admiral John Rodgers, demanded an official apology from the Regent within ten days. When no apology was offered, the Americans attacked on June 10, 1871.
The US sailors and Marines landed on Gangwha Island and systematically destroyed all five of the garrison forts and cannon batteries there. It was a one-sided battle; the US was armed with modern rifles and howitzer guns, while the Koreans had matchlock muskets and muzzle-loading cannon. Within two days, the force of US Marines and sailors had captured the entire island. Almost 250 Koreans had been killed, and three Americans. Nine Sailors and three Marines were awarded the Medal of Honor--the first to be awarded for actions outside of the US.
After the battle, the American fleet offered again to negotiate a treaty, trying to use the five wounded Koreans it had captured as a bargaining chip. In response, the Korean Regent notified the US that it could keep the wounded prisoners if it wanted them, and there would be no treaty. On July 3, its diplomatic mission a total failure, the US fleet pulled anchor and left.
The Joseon Dynasty continued its policy of xenophobic isolationism for the next five years, until, in 1876, a powerful Japanese fleet sailed into the Han River and forced Korea to sign a treaty opening up three port cities to Japanese ships. Korea became a Japanese colony and was formally annexed in 1910. With Japan's defeat in 1945, Korea became independent, but was divided by the UN into two zones. On June 25, 1950, the Communist North Korea invaded the pro-US South Korea and US troops were sent to stop the invasion, sparking what was actually the second Korean War.