The UN offensive into North Korea was a large-scale offensive in late 1950 by United Nations (UN) forces against North Korean forces during the Korean War.
On 27 September near Osan, UN forces coming from Inchon linked up with UN forces that had broken out of the Pusan Perimeter and began a general counteroffensive. The North Korean Korean People's Army (KPA) had been shattered, and its remnants were fleeing back towards North Korea. The UN Command then decided to pursue the KPA into North Korea, completing their destruction and unifying the country. On 30 September Republic of Korea Army (ROKA) forces crossed the 38th parallel, the de facto border between North and South Korea on the east coast of the Korean peninsula, and this was followed by a general UN offensive into North Korea. Within one month UN forces were approaching the Yalu River, prompting Chinese intervention in the war. Despite the initial Chinese attacks in late October – early November, the UN renewed their offensive on 24 November before it was abruptly halted by massive Chinese intervention in their Second Phase Offensive starting on 25 November.
Background North Korea repelled North Korea invaded South Korea at the end of June 1950 in Operation Pokpung. By late-July, South Korea was reduced to the southeast tip of the peninsula around Pusan, held by the United Nations Command (UNC) forces, mainly the US Eighth Army, and the ROKA. The Korean People's Army (KPA) offensive on the Pusan perimeter in August and September failed to break through; the KPA had suffered heavy casualties since the start of the war and was overextended. By September, the KPA was attacking Pusan with 70,000 troops, while the UNC had 140,000 troops with materiel superiority. The US estimated that there were 90,000 KPA troops in South Korea. In mid-September, US X Corps, composed of US 1st Marine Division and US 7th Infantry Division, made a surprise landing at Inchon in the North Korean rear west of Seoul; UNC commander General Douglas MacArthur accompanied the landing. Shortly afterward, the UNC and ROKA broke out of Pusan. The KPA began a general retreat from South Korea to avoid encirclement, giving little resistance to their rapidly advancing pursuers from Pusan. Eighth Army advanced in the west and linked up with X Corps at Osan on 26 September. The ROKA advanced in the east and reached the 38th parallel by the end of the month.
Deciding to invade North Korea MacArthur intended to invade North Korea even though he did not have the authority to do so. The pause in operations was for logistical purposes. However, on 27 September he received authorization to proceed from the US Joint Chiefs of Staff (JCS). The JCS directed MacArthur to pursue and destroy North Korean forces up to the Chinese and Soviet borders; employment of UN forces in the border areas was to be avoided as a "matter of policy". Air and naval operations on Chinese or Soviet territory were forbidden. Furthermore, as a precondition, no advance beyond the 38th parallel was permitted if a major China or Soviet military intervention had occurred or was likely to. A further message on 29 September explicitly informed MacArthur that an invasion was authorized by the US government.
The UNC had fulfilled United Nations Security Council (UNSC) Resolution 82 by repelling the North Korean invasion. Now, a military reunification of Korea seemed possible, but Soviet opposition prevented UNSC Resolution 83, concerning the restoration of "international peace and security in the area", from being used as justification. The United States and the United Kingdom sought broad support for military reunification by submitting Resolution 376(V) to the UN General Assembly on 29 September; this circumvented the Soviet veto in the UNSC. The resolution was adopted on 7 October, giving the UNC invasion of North Korea an official UN mandate just as it began.
Chinese intentions The US believed Chinese and Soviet intervention to be unlikely; the expected intervention following the reversal in mid-September had not occurred. Intelligence from diplomats in China — including K. M. Panikkar, the ambassador of India to China — suggested that China was too preoccupied with internal affairs and gaining UN membership to intervene; it was not until 27 September that the US started receiving intelligence from these sources to the contrary. At the end of the month, India refused to cosponsor Resolution 376(V) and an UNC offensive into North Korea on the grounds that it would cause Chinese intervention.
China was asked by the Soviet Union to intervene after the Inchon landing, and North Korea repeated the request on 1 October; on 2 October, China — with strong support from Mao Zedong and Peng Dehuai — agreed to intervene on the condition that the Soviet Union provided materiel aid. The inability of Western intelligence to determine Chinese intentions earlier was caused by the late decision and the resulting lack of strong warning signals until late-September.
United Nations preparations Planning MacArthur received the first outline for the attack plan from US Far East Command (FEC) on 26 September. In the plan, Eighth Army advanced northward on the west flank from Seoul, and X Corps advanced westward after landing at Wonsan on the east coast. X Corps move west would block the KPA retreating from South Korea, and complete the encirclement and capture of Pyongyang with Eighth Army. In the outline, the Wonsan landing occurred 12 days after Eighth Army passed through X Corps in the Seoul-Inchon area, and three to seven days after the start of Eighth Army attack. MacArthur approved this plan on 29 September.
Wonsan was the principal port and a major transportation hub on Korea's east coast. It was a North Korean naval base, and Soviet materiel shipped by sea from Vladivostok arrived through it. Korean petroleum refining was centered there and the important industrial Hamhung-Hungnam area was to the north. A railroad went northeast to Vladivostok, and a railroad and the "only reasonably good" lateral road went west toward Pyongyang slightly north of the 39th parallel.
Separating Eighth Army and X Corps into different field commands and geographically was controversial. General Walton Walker, commander of Eighth Army, and General Doyle O. Hickey, FEC's Deputy Chief of Staff, supported subordinating X Corps to Eighth Army, and starting the offensive two weeks earlier by having X Corps lead Eighth Army northward; the ROKA had met little resistance on the east flank from Pusan and its overland advance would likely reach Wonsan before an amphibious assault by X Corps. Admiral C. Turner Joy, commander of US Naval Forces Far East, believed X Corps could capture Wonsan faster by advancing overland from Seoul. X Corps leading the way and doing most of the fighting until Pyongyang would limit supply consumption on the west flank. The east flank could be supplied by sea after capturing the ports of Wonsan and Hungnam. The alternate plan was developed by Eighth Army staff but never proposed. General O.P. Smith, commander of 1st Marine Division, was also skeptical of using his division in northeast Korea.
MacArthur's decision to split the formations may have been due to logistical or leadership concerns. Korean transportation infrastructure was heavily damaged by the war and, combined with most principal routes being in the north–south direction due to the mountainous terrain, he may have believed supplying a quick advance by both Eighth Army and X Corps from Inchon to be impossible. MacArthur may also have lacked confidence in Walker's ability to control so large a force. Coordinating the two formations may not have appeared to be a serious problem, as the base of operations for Korea was in Japan.
X Corps' preparations Joint Task Force 7 was reestablished, with Admiral Arthur Dewey Struble in command, to transport X Corps and provide the corps with naval support for and after the landing. General Edward Almond, commander of X Corps, and Struble received preliminary instructions from FEC on 1 October. MacArthur formalized the attack on 2 October in United Nations Operation Order 2. The port at Inchon was congested by adverse tidal conditions, the quantity of amphibious craft available, and its concurrent use by Eighth Army. MacArthur decided to load part of the force at Pusan to meet the two week schedule. 1st Marine Division, the amphibious assault element of X Corps, had priority for loading at Inchon. X Corps headquarters would also load at Inchon, while the US 7th Infantry Division and most of X Corps moved to Pusan for loading. During the attack, 1st Marine Division would secure the corps' bridgehead, with 7th Infantry Division advancing west and linking up with Eighth Army at Pyongyang.
1st Marine Division was informed of the planned landing at Wonsan on 30 September. On 1 October, it was ordered to submit plans by 3 October for a 15 October landing; this was impossible as ships had yet to be allocated. On 7 October, Admiral Doyle, commander of the Attack Force, recommended a landing date of no earlier than 20 October; the amphibious assault forces could meet no earlier. MacArthur tentatively accepted the new date on the condition that every effort was made to meet 15 October. Joint Task Force 7's operation plan and organization was published on 9 October.
UN Operations Plan 2 was put into effect on 10 October.
1st Marine Division embarkation at Inchon On 3 October, X Corps ordered 1st Marine Division to assemble at Inchon, which was mostly completed by 6 October. The division had 28,147 men, including attachments of 40 US Army troops and 4,516 Korean Marines.
Loading was made more difficult by tides, mud banks at low tide, and limited port facilities. The staging area was inadequate. Only seven Landing Ship, Tank (LSTs) or landing craft could beach at a time, and only at high tide. There was only one small pier to load vehicles onto Landing Craft Mechanized, again only at high tide. Vehicles were ferried to attack transports and amphibious cargo ships on LSTs, and the vehicles moved by cranes from the top decks of the landing craft to the ships. Bulk cargo was ferried out from the tidal basin. X Corps requested that the port halt any daytime unloading that was unrelated to the corps, otherwise loading would be delayed for an estimated six to twenty days. The 1st Marine Division was informed that 10 days of supplies would be bottom loaded, but this was not done and resulted in ships being reloaded. In reloading, X Corps drew considerably from supply dumps that could have been left for Eighth Army. Additional rations were flown in from Japan via Kimpo Airfield. Troops began embarking on 9 October. X Corps headquarters relocated to on 11 October. Bulk cargo loading began on 8 October and continued until 16 October when all X Corps loading at Inchon was completed.
Amphibious Group One and the LSTs left Inchon late in the afternoon of 16 October. The main body of the Attack Force, carrying 1st Marine Division, left Inchon on the morning of 17 October. After arriving off of Wonsan, the flotilla steamed slowly back and forth from 19 to 25 October in the Sea of Japan just outside the Wonsan channel; the restless Marines called it "Operation Yo-yo".
7th Infantry Division embarkation at Pusan