Fighting as part of EUSAK, by this time fanned out throughout North Korea, the 1st Marine Division did not meet the expected NKPA resistance. Instead, large-scale Chinese Communist Forces had entered the war. As X Corps swept north toward the Yalu River in November 1950, the Marines became the first United States troops to defeat the Chinese Communists in battle. At Sudong, after four days of savage fighting, the Marine RCT-7 so badly crippled major elements of the 124th CCF Division that it was never again committed as an organic unit.
When the Chinese forces struck in full force at the Chosin Reservoir, X Corps units were forced back. Elements of a nine-division assault force, the CCF 9th Army Group, which had been sent into Korea with the specific mission of annihilating the 1st Marine Division, began to attack. On 27 November, the Chinese directed a massive frontal assault against 5th and 7th Marines positions at Yudam-ni, west of the reservoir. Another CCF division, moving up from the south, cut the MSR held by the 1st Marines so that the division at Yudam-ni, west of the reservoir, was completely encircled by Communist forces. Many experts considered the 1st Marine Division as lost. Others thought the only way to save it was to airlift it out, leaving its equipment behind. Instead, the Marines seized the initiative at Yudam-ni and cut a path through CCF units blocking a route to Hagaru. The division battled its way out in 20-degree-below-zero weather 78 miles over icy, winding mountain roads from the reservoir to the Hamhung-Hungnam area where, on 15 December, it redeployed to South Korea.
Integrated ground and air action enabled the 10,000 Marines and attached 4,000 Army-Royal Marine troops to break out of the entrapment and move south. During 13 tortuous days the Marines had withstood hostile strength representing elements of six to eight CCF divisions. The major result, from the military view, was that the Marine division properly evacuated its dead and wounded, brought out all operable equipment, and completed the retrograde movement with tactical integrity.
Not only had the Chinese (with a total of 60,000 men in assault or reserve) failed to accomplish their mission, destruction of the division, but the Marine defenders had dealt a savage blow to the enemy in return. POW debriefings later revealed that assault units of the CCF 9th Army Group had been rendered so militarily ineffective that nearly three months were required for its replacement, re-equipment, and reorganization.
Early in 1951, the 1st Marine Division was reassigned to IX Corps for Operation KILLER, a limited offensive ordered by the EUSAK Commander, General Matthew B. Ridgway. In Operation RIPPER, in March, the division led another IX Corps advance as it drove toward the 38th Parallel on the east-central front. When the Chinese struck back with their spring offensive on 22 April, the Marines were transferred to operational control of X Corps and counterattacked to restore the UNC defensive position in the far eastern sector. During May and June, the 1st Marine Division continued to punish the enemy in the Punchbowl area of eastern Korea, driving the CCF back to Yanggu and the Soyang River corridor.
Activity all along the UNC front came to an uncertain halt in July 1951 when Allied and Communist negotiators met at Kaesong for truce talks initiated by the enemy. In August the MLR flared into action again, and the Marine Division was engaged in new counterthrusts in the Punchbowl area. Fighting during the next three weeks involved the division in some of its hardest offensive operations in Korea. It also developed that this would be the last offensive for the Marines. In November 1951, as a result of the truce talks and possibility of ending hostilities, General Ridgway, now UNC Commander, ordered the Eighth Army to cease offensive operations and begin an active defense of the front.
The war of fire and movement had turned into one of positional warfare, a defensive posture by UN forces that would continue for the last 21 months of the three-year conflict. Throughout the winter of 1951–1952, the Marines conducted vigorous patrol activities in their sector of X Corps. Although it was a lackluster period of trench warfare for the average infantryman, major tactical innovations were being pioneered by the division with its use of the transport helicopter for logistical and resupply missions.
In March 1952, the 1st Marine Division was transferred from the eastern X Corps line 140 miles west to strengthen the far end of the Eighth Army MLR in the I Corps sector. The division was relocated in the path of the enemy’s invasion route to Seoul, where weak defenses in the Kimpo coastal area had threatened the security of the UNC front. Here the division’s four infantry regiments (including the 1st Korean Marine Corps RCT) held nearly 35 miles of front line in the critical Panmunjom-Munsan area. The demilitarized route for the United Nations negotiators led through the Marine lines. It was the most active sector of the UN front for the next 16 months. This key position guarded the best routes of advance from North Korea to Seoul and indicated the high regard in which General James A. Van Fleet, EUSAK commander, held the Marines.
West Korean terrain was rugged, hilly, and friendly to the CCF who had the advantage of high ground positions as well as considerably more manpower. Although cast in an unaccustomed defensive warfare role, rather than a true attack mission, the Marines repelled an almost continuous series of enemy probes. While truce talks went on at nearby Panmunjom, fighting as furious as at any time earlier in the war flared up intermittently as the CCF tried to gain additional terrain for bargaining purposes. During 1952–1953, the Marine division beat off determined CCF limited objective attacks on Bunker Hill, the Hook, Vegas, and Boulder City outposts up until—literally—the final day of the war, 27 July 1953.
In reviewing Marine actions during this period, the Secretary of the Navy commented: