The Civil War
When in March of 1862 the Army of the Potomac, then under General McClellan was reorganized into four corps by President Lincoln's directive, I U.S. Corps was born. General McDowell was the first commander of the Corps that was to see many other illustrious commanders. Troops of the Corps took part in the Battle of Manassas (Second Bull Run), and saw action at Antietam, at Fredericksburg, in the Chancellorsville Campaign, and at the decisive Battle of Gettysburg. Its troops served until, in a later reorganization of the Army of the Potomac, its depleted divisions were consolidated and transferred to V Corps and it was deactivated with the reduction of that Army to three Corps.
The Spanish-American War
The Corps was reactivated in 1898 at Camp Thomas, Chickamauga, Georgia, immediately after the United States declared war on Spain. In July of that year, under the command of Major General John R. Brooke, elements of the I U.S. Corps were landed at Anayo on 31 July to take part in the Campaign of Puerto Rico. The force advanced to Guayam, where it engaged the Spanish on 5 August. An advance to Cayey followed in preparation for a major attack; however, before it was launched, the Armistice was consummated.
World War I
I Corps was activated at Neufchateau, France, 15 January 1918. In July it helped stop a German drive, and counterattacked in the Aisne-Marne offensive. The Corps attacked the southern face of the St. Mihiel Salient in September and carried all its objectives. I Corps was transferred to the Meuse-Argonne front on 20 September where it took part in the last major offensive of the war, fighting through the Argonne Forest. I Corps was demobilized at Tonnerre, France, in March 1919.
World War II
On 1 November 1940, I Corps was reactivated. It moved to Australia in August 1942 where it prepared for action in New Guinea. During the fall and winter of 1942, I Corps fought in the Buna-Gona battles on New Guinea. In April 1944, an I Corps task force made an amphibious assault at Hollandia. When the campaign ended, the 18th Japanese Army had been isolated. By 24 June, I Corps units had secured the Island of Biak off the New Guinea Coast. With New Guinea secure, attention turned to the Philippines.
I Corps was next assigned an amphibious landing on Luzon. It landed at Lingayen Gulf and, in a month-long advance, pushed across the island to split the Japanese forces. The Corps then turned north and drove two hundred miles to break the Japanese hold on northern Luzon. A war's end, the Corps was preparing to lead an assault in the invasion of Japan.
After the surrender in 1945, the mission became one of occupation of Japan. I Corps remained in Japan until 1950 when it was inactivated.
The Korean War
The Korean War broke out three months later. On 2 August 1950, I Corps was reactivated, and by 27 August the Corps had assumed command of units in the Pusan Perimeter. I Corps led an offensive launched from the perimeter. By 1 October, the Corps had driven past Seoul and beyond the 38th Parallel. On the 29th it took the North Korean Capital, and by month's end the Corps was only a few miles from the border to China.
The Chinese entered the War, forcing all UN Forces to withdraw. They re-took Seoul and pushed on south in late 1950. I Corps counterattacked and participated in the seesaw battles that swept back and forth around the 38th Parallel for the next year. Periods of relative quiet and heavy fighting continued at such places as Pork Chop Hill, Old Baldy, and The Hook for the next two years, until the Armistice was signed. I Corps remained in Korea until 1971 when the Corps Headquarters was reduced to zero strength.
In each of the these wars there is a striking similarity. In each the Corps entered the fighting when things were going badly, and in each the Corps performed its mission, emerging victorious. It has participated in more campaigns than any other corps and is the only corps ever to receive the U.S. Presidential Unit Citation. First Corps is the most decorated corps in the active Army.
I Corps was transferred to Fort Lewis, Washington, on 1 October 1981, where it was brought back to strength. Designated an early deploying corps for military contingencies in the Pacific, I Corps is now able to deploy on short notice with both Active Army and Reserve Component forces.