The Eighth Army is a U.S. field army which commands all United States Army forces in South Korea. It is headquartered at the Camp Humphreys in the Anjeong-ri of Pyeongtaek, South Korea. Eighth Army relocated its headquarters from Yongsan to Camp Humphreys in the summer of 2017. It is the only field army in the U.S. Army. It is responsible to United States Forces Korea and United States Army, Pacific.

The unit first activated on 10 June 1944 in the United States, under the command of Lieutenant General Robert L. Eichelberger. The Eighth Army took part in many of the amphibious landings in the Southwest Pacific Theater of World War II, eventually participating in no less than sixty of them. The first mission of the Eighth Army, in September 1944, was to take over from the U.S. Sixth Army in New Guinea, New Britain, the Admiralty Islands and on Morotai, in order to free up the Sixth Army to engage in the Philippines Campaign (1944–45).

The Eighth Army again followed in the wake of the Sixth Army in December 1944, when it took over control of operations on Leyte Island on 26 December. In January, the Eighth Army entered combat on Luzon, landing the XI Corps on 29 January near San Antonio and the 11th Airborne Division on the other side of Manila Bay two days later. Combining with I Corps and XIV Corps of Sixth Army, the forces of Eighth Army next enveloped Manila in a great double-pincer movement. Eighth Army's final operation of the Pacific War was that of clearing out the southern Philippines of the Japanese Army, including on the major island of Mindanao, an effort that occupied the soldiers of the Eighth Army for the rest of the war.

Eighth Army was to have participated in Operation Downfall, the invasion of Japan. It would have taken part in Operation Coronet, the second phase of the invasion, which would have seen the invasion of the Kantō Plain on eastern Honshū. However, the Japanese surrender cancelled the invasion, and the Eighth Army found itself in charge of a peaceful occupation. Occupation forces landed on 30 August 1945, with its headquarters in Yokohama, then the HQ moved to the Dai-Ichi building in Tokyo.

At the beginning of 1946, Eighth Army assumed responsibility for occupying all of Japan. Between February 1946 and October 1949, the Eighth Army tried 996 suspected Japanese war criminals in the Yokohama War Crimes Trials, convicting 854. These trials constituted the majority of American war crimes trials in the Asia-Pacific region. Four quiet years then followed, during which the Eighth Army gradually transitioned from a combat-ready fighting force into a constabulary. Lieutenant General Walton H. Walker took command in September 1948, and he tried to re-invigorate the Army's training, with mixed success.

In June 1950 75,000 North Korean Korean People's Army (KPA) troops with Soviet made tanks invaded South Korea, igniting the Korean War. U.S. naval and air forces quickly became involved in combat operations, and it was soon clear that U.S. ground forces would have to be committed. To stem the North Korean advance, the occupation forces in Japan were thus shipped off to South Korea as quickly as possible, but their lack of training and equipment was telling, as some of the initial U.S. units were destroyed by the KPA. However, the stage was eventually reached as enough units of Eighth Army arrived in Korea to make a firm front. The KPA threw themselves against that front, the Pusan Perimeter, and failed to break it.

Eighth Army arrived in July 1950 and never left. —Lt. Gen. Thomas S. Vandal, CG, Eighth Army, 29 August 2017

In the meantime, Eighth Army had reorganized, since it had too many divisions under its command for it to exercise effective control directly. The I Corps and the IX Corps were reactivated in the United States and then shipped to Korea to assume command of Eighth Army's subordinate divisions.