X Corps was a unit of the United States Army that took part in the invasion of Leyte under Sixth Army during 1944.

During the Korean War, it took part in Operation Chromite, the landings at Inchon. At Inchon it had the 1st Marine Division and 7th Infantry Division under its command. The Marines first captured an island offshore of Inchon as a prelude to the assault and at the next tide, the main attack went in. Despite the noise of the attack on the offshore island, it completely surprised the North Korean forces.

Inchon had a very large tidal range, and was thus very risky to use as a landing site. Nonetheless, the landing proved to be an enormous success. It is by far the largest amphibious assault to have taken place since World War II, and one of the boldest.

After the landing at Inchon, X Corps initially attacked up the Korean peninsula on the left wing of Eighth Army. However, it was withdrawn from the line relatively shortly thereafter to prepare for another amphibious assault, this time at Wonsan on the eastern coast. This action proved to be a mistake on two counts. First, forces of the Eighth Army moving by land reached the town before the assault went in. Second, it proved to be too far for UN forces to go. After landing at Wonsan, X Corps advanced inland towards Chosin Reservoir. The Chinese chose this moment to intervene in the war in force. X Corps was strung out along many miles, and under the Chinese attacks some units disintegrated. However, the 1st Marine Division, the unit furthest forward, maintained cohesion, and began its famous fight to the sea from Chosin. Completely surrounded, it successfully engaged seven or eight Chinese division and managed to reach the relative safety of the coast, where it was evacuated.

After the retreat from Chosin, once its units had been put back on an even keel, X Corps went into the line, and remained there for the rest of the war. It is widely contended that X Corps remained outside of the direct command of Eighth Army too long. X Corps reporting directly to the Supreme Commander had been necessary for the Inchon landings and still defensible for the Wonsan attack. However, after it entered the main line, conventional military doctrine indicated that it should have been placed immediately under the command of Eighth Army. General Douglas MacArthur, the commander of the United States forces, was accused of favouritism towards Edward Almond, the commander of X Corps, who was his former chief of staff and a personal friend.

After X Corps was placed under the command of Eighth Army, it performed well for the rest of the war.