Originally formed in the Regular Army in 1918 but not in time to see action in World War, the IX Corps was demobilized from 1919 to 1921, when it was reconstituted in the Organized Reserves as Headquarters, IX Corps. Though activated more than a year prior to U.S. entry into World War I, the Corps first spent its time training other units that would carry out the actual landings, such as Tenth Army (invasion of Okinawa) and Sixth Army (slated invasion of Japan). Consequently, the IX Corps was awarded a World War II Asiatic-Pacific Theater without a streamer.
During the Korean War, on the other hand, the IX Corps arrived very early in the Korean War, missing only the opportunity to take part in the initial scattered defensive actions that failed to slow the North Korean onslaught. After halting the North Korean steamroller at the Pusan Perimeter, IX Corps advanced with U.S. and U.N. forces through South Korea, over the Yalu River and into North Korea itself, only to be flung back across the border into South Korea when hundreds of thousands of Chinese Soldiers counterattacked in late November. IX Corps would go on to take part in every remaining campaign of the conflict (nine out of ten in all) and earned two Republic of Korea Presidential Unit Citations.
The Korean War would mark the last combat for the IX Corps, which had components stripped away and assigned to various commands in the Pacific. It was inactivated in November 1995.