The corps spent most of World War II in charge of defenses on the West Coast of the United States, before moving to Hawaii and Leyte to plan and organize operations for US forces advancing across the Pacific.

[3] IX Corps was subordinate to Second United States Army, and after moving its headquarters to Saint-Mihiel, and commanded forces along the armistice line between Jonville-en-Woëvre and Fresnes-en-Woëvre until its deactivation.

An oddity was that only the VIII and IX Corps were considered direct lineal descendants of their World War I predecessors.

Concurrently, the corps’ designated headquarters location for peacetime organization purposes was transferred from San Francisco to Seattle, Washington.

Reserve personnel assigned to the corps headquarters attended summer training at the Presidio of San Francisco.

[8] The IX Corps headquarters was activated on 24 October 1940, less Reserve personnel, at Fort Lewis where it assumed command and control of the 3rd and 41st Divisions.

[10] The corps oversaw defenses on the West Coast for the majority of the war, but in 1944 it was moved to Fort McPherson, Georgia in preparation for deployment overseas.

[10] When it arrived in Hawaii, IX Corps was put under the command of the Tenth United States Army.

[11] In Leyte, the corps was tasked with the planning of Operation Downfall, the invasion of mainland Japan, specifically the island of Kyushu.

[11] The next few years were a period during which the terms of the surrender were supervised and enforced; Japanese military installations and material were seized, troops were disarmed and discharged, and weapons of warfare disposed of.

The Eighth Army, taking charge of the conflict, requested the activation of three corps headquarters for its growing command of UN forces.

[21] Four days later I Corps troops began a general offensive northward against crumbling KPA opposition to establish contact with forces of the 7th Infantry Division driving southward from the beachhead.

Major elements of the KPA were destroyed and cut off in this aggressive penetration; the link-up was effected south of Suwon on 26 September.

The Chinese People's Volunteer Army (PVA) entered the war on the side of North Korea, making their first attacks in late October.

By the end of October the city of Chongju, 40 miles (64 km) from the Yalu River border of North Korea, had been captured.

[26] By 1 December, the division was almost completely destroyed; it lost virtually all of its heavy equipment and vehicles, as well as suffering 4,940 men killed or missing.

[27] The 25th Infantry Division, on its western flank, was also hit by overwhelming PVA forces of the 39th Army Corps, facing strong attacks and suffering heavy casualties and losses in equipment in the Battle of Ch'ongch'on River.

[25] The Eighth Army suffered heavy casualties, ordering a complete withdrawal to the Imjin River, south of the 38th Parallel, having been destabilised by the overwhelming PVA forces.

[32][33] The corps were to advance steadily northward, protected by heavy artillery and close air support, until they captured Seoul.

However, it encountered stiff resistance from PVA forces dug into the hilly country around Chipyong-ni and was still bogged down in combat by 2 February.

PVA forces had established machine gun nests in the hillside and mined roads to slow the corps' advance.

[36] Between February and March, the corps participated in Operation Killer, pushing PVA forces north of the Han River.

[43][44] In September, the UN Forces launched another counteroffensive with the 24th Infantry Division at the center of the line, west of the Hwachon Reservoir.

[18] In October 1952, PVA forces conducted a large offensive against IX Corps' sector, against the hilly countryside around the Iron Triangle region of Cheorwon, Kumhwa, and Pyongyang.

The PVA 8th Field Army sent heavy assaults against the ROK forces guarding Hill 395 in the Battle of White Horse.

[48] At the same time, PVA forces attacked Arrowhead Hill, which was held by the 2nd Infantry Division 2 miles (3.2 km) away.

Both hills changed hands several times, but after two weeks and almost 10,000 casualties, the PVA were unsuccessful in capturing either objective and withdrew.

[53] For the remainder of the year, UN and PVA forces both conducted a series of smaller raids on each other's lines, avoiding major conflicts, as armistice negotiations continued unsuccessfully.

A major change in the Army's organization in the Pacific occurred on 15 May 1972, in conjunction with the return of Okinawa to Japanese control after twenty-seven years of administration by the United States.

IX Corps remained a command component of United States Army Japan until 1994, when it was inactivated.