The Korean War (in South Korean Hangul: 한국전쟁, Hanja: 韓國戰爭, Hanguk Jeonjaeng, "Korean War"; in North Korean Chosungul: 조국해방전쟁, Joguk Haebang Jeonjaeng, "Fatherland Liberation War"; 25 June 1950 – 27 July 1953)[a] was a war between North and South Korea, in which a United Nations force led by the United States of America fought for the South, and China fought for the North, which was also assisted by the Soviet Union. The war arose from the division of Korea at the end of World War II and from the global tensions of the Cold War that developed immediately afterwards.
Korea was ruled by Japan from 1910 until the closing days of World War II. In August 1945, the Soviet Union declared war on Japan and—by agreement with the United States—occupied Korea north of the 38th parallel. U.S. forces subsequently occupied the south and Japan surrendered. By 1948, two separate governments had been set up. Both governments claimed to be the legitimate government of Korea, and neither side accepted the border as permanent. The conflict escalated into open warfare when North Korean forces—supported by the Soviet Union and China—invaded South Korea on 25 June 1950. On that day, the United Nations Security Council recognized this North Korean act as invasion and called for an immediate ceasefire. On 27 June, the Security Council adopted S/RES/83 : Complaint of aggression upon the Republic of Korea and decided the formation and dispatch of the UN Forces in Korea. Twenty-one countries of the United Nations eventually contributed to the defense of South Korea, with the United States providing 88% of the UN's military personnel.
After the first two months of the conflict, South Korean forces were on the point of defeat, forced back to the Pusan Perimeter. In September 1950, an amphibious UN counter-offensive was launched at Inchon, and cut off many of the North Korean attackers. Those that escaped envelopment and capture were rapidly forced back north all the way to the border with China at the Yalu River, or into the mountainous interior. At this point, in October 1950, Chinese forces crossed the Yalu and entered the war. Chinese intervention triggered a retreat of UN forces which continued until mid-1951. After these dramatic reversals of fortune, which saw Seoul change hands four times, the last two years of conflict became a war of attrition, with the front line close to the 38th parallel. The war in the air, however, was never a stalemate. North Korea was subject to a massive bombing campaign. Jet aircraft were used in air-to-air combat for the first time in history, and Soviet pilots covertly flew in defense of their Communist allies.
The fighting ended on 27 July 1953, when the armistice was signed. The agreement created the Korean Demilitarized Zone to separate North and South Korea, and allowed the return of prisoners. Clashes have continued to the present.
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In the U.S., the war was initially described by President Harry S. Truman as a "police action" as it was an undeclared military action, conducted under the auspices of the United Nations. It has been referred to in the Anglosphere as "The Forgotten War" or "The Unknown War" because of the lack of public attention it received both during and after the war, and in relation to the global scale of World War II, which preceded it, and the subsequent angst of the Vietnam War, which succeeded it.
In China, the war is officially called the "War to Resist U.S. Aggression and Aid Korea" (simplified Chinese: 抗美援朝战争; traditional Chinese: 抗美援朝戰爭; pinyin: Kàngměiyuáncháo zhànzhēng), although the term "Chaoxian (Korean) War" (simplified Chinese: 朝鲜战争; traditional Chinese: 朝鮮戰爭; pinyin: Cháoxiǎn zhànzhēng) is also used in unofficial contexts, along with the term "Korean Conflict" (simplified Chinese: 韩战; traditional Chinese: 韓戰; pinyin: Hán Zhàn) more commonly used in regions such as Hong Kong and Macau.
Japan destroyed the influence of China over Korea in the First Sino-Japanese War (1894–95), ushering in the short-lived Korean Empire. A decade later, after defeating Imperial Russia in the Russo-Japanese War (1904–05), Japan made Korea its protectorate with the Eulsa Treaty in 1905, then annexed it with the Japan–Korea Annexation Treaty in 1910.
Many Korean nationalists fled the country. A Provisional Government of the Republic of Korea was founded in 1919 in Nationalist China. It failed to achieve international recognition, failed to unite nationalist groups, and had a fractious relationship with its American-based founding President, Syngman Rhee. From 1919 to 1925 and beyond, Korean Communists led internal and external warfare against the Japanese.
Korea was considered to be part of the Empire of Japan as an industrialized colony along with Taiwan, and both were part of the Greater East Asia Co-Prosperity Sphere. In 1937, the colonial Governor-General, General Jirō Minami, commanded the attempted cultural assimilation of Korea's 23.5 million people by banning the use and study of Korean language, literature, and culture, to be replaced with that of mandatory use and study of their Japanese counterparts. Starting in 1939, the populace was required to use Japanese names under the Sōshi-kaimei policy. Conscription of Koreans for labor in war industries began in 1939, with as many as 2 million Koreans conscripted into either the Japanese Army or into the Japanese labor force.
In China, the Nationalist National Revolutionary Army and the Communist People's Liberation Army helped organize refugee Korean patriots and independence fighters against the Japanese military, which had also occupied parts of China. The Nationalist-backed Koreans, led by Yi Pom-Sok, fought in the Burma Campaign (December 1941 – August 1945). The Communists, led by Kim Il-sung among others, fought the Japanese in Korea and Manchuria.
During World War II, Japan used Korea's food, livestock, and metals for their war effort. Japanese forces in Korea increased from 46,000 soldiers in 1941 to 300,000 in 1945. Japanese Korea conscripted 2.6 million forced laborers controlled with a collaborationist Korean police force; some 723,000 people were sent to work in the overseas empire and in metropolitan Japan. By 1942, Korean men were being conscripted into the Imperial Japanese Army. By January 1945, Koreans made up 32% of Japan's labor force. At the end of the war, other world powers did not recognize Japanese rule in Korea and Taiwan.
At the Tehran Conference in November 1943 and the Yalta Conference in February 1945, the Soviet Union promised to join its allies in the Pacific War within three months of the victory in Europe. Accordingly, it declared war on Japan on 9 August 1945. By 10 August, the Red Army had begun to occupy the northern part of the Korean peninsula.
On the night of 10 August in Washington, American Colonels Dean Rusk and Charles H. Bonesteel III were tasked with dividing the Korean Peninsula into Soviet and U.S. occupation zones and proposed the 38th parallel. This was incorporated into America's General Order No. 1 which responded to the Japanese surrender on 15 August. Explaining the choice of the 38th parallel, Rusk observed, "even though it was further north than could be realistically reached by U.S. forces, in the event of Soviet disagreement...we felt it important to include the capital of Korea in the area of responsibility of American troops". He noted that he was "faced with the scarcity of US forces immediately available, and time and space factors, which would make it difficult to reach very far north, before Soviet troops could enter the area". As Rusk's comments indicate, the Americans doubted whether the Soviet government would agree to this. Stalin, however, maintained his wartime policy of co-operation, and on 16 August the Red Army halted at the 38th parallel for three weeks to await the arrival of U.S. forces in the south.
On 8 September 1945, U.S. Lt. Gen. John R. Hodge arrived in Incheon to accept the Japanese surrender south of the 38th parallel. Appointed as military governor, General Hodge directly controlled South Korea as head of the United States Army Military Government in Korea (USAMGIK 1945–48). He established control by restoring to power the key Japanese colonial administrators, but in the face of Korean protests he quickly reversed this decision. The USAMGIK refused to recognize the provisional government of the short-lived People's Republic of Korea (PRK) because it suspected it was communist.
In December 1945, Korea was administered by a U.S.-Soviet Union Joint Commission, as agreed at the Moscow Conference (1945), with the aim of granting independence after a five-year trusteeship. The idea was not popular among Koreans and riots broke out. To contain them, the USAMGIK banned strikes on 8 December 1945 and outlawed the PRK Revolutionary Government and the PRK People's Committees on 12 December 1945.
The right-wing Representative Democratic Council, led by Syngman Rhee, who had arrived with the U.S. military, opposed the trusteeship, arguing that Korea had already suffered from foreign occupation far too long. General Hodge began to distance himself from the proposal, even though it had originated with his government.
On 23 September 1946, an 8,000-strong railroad worker strike began in Pusan. Civil disorder spread throughout the country in what became known as the Autumn uprising. On 1 October 1946, Korean police killed three students in the Daegu Uprising; protesters counter-attacked, killing 38 policemen. On 3 October, some 10,000 people attacked the Yeongcheon police station, killing three policemen and injuring some 40 more; elsewhere, some 20 landlords and pro-Japanese South Korean officials were killed. The USAMGIK declared martial law.
Citing the inability of the Joint Commission to make progress, the U.S. government decided to hold an election under United Nations auspices with the aim of creating an independent Korea. The Soviet authorities and the Korean Communists refused to co-operate on the grounds it would not be fair, and many South Korean politicians also boycotted it. A general election was held in the South on 10 May 1948. It was marred by terrorism and sabotage resulting in 600 deaths. North Korea held parliamentary elections three months later on 25 August.
The resultant South Korean government promulgated a national political constitution on 17 July 1948, and elected Syngman Rhee as President on 20 July 1948. The Republic of Korea (South Korea) was established on 15 August 1948. In the Russian Korean Zone of Occupation, the Soviet Union established a Communist North Korean government led by Kim Il-sung. President Rhee's régime excluded communists and leftists from southern politics. Disenfranchised, they headed for the hills, to prepare for guerrilla war against the US-sponsored ROK Government.
Meanwhile, on 3 April 1948, what began as a demonstration commemorating Korean resistance to Japanese rule ended with the Jeju Uprising where between 14,000 and 60,000 people died. South Korean soldiers carried out large scale atrocities during the suppression of the uprising. In October 1948, some South Korean soldiers mutinied against the clampdown in the Yeosu-Suncheon Rebellion.
The Soviet Union withdrew as agreed from Korea in 1948, and U.S. troops withdrew in 1949. On 24 December 1949, South Korean forces killed 86 to 88 people in the Mungyeong massacre and blamed the crime on marauding communist bands. By early 1950, Syngman Rhee had about 30,000 alleged communists in jails and about 300,000 suspected sympathizers enrolled in the Bodo League re-education movement.
With the end of the war with Japan, the Chinese Civil War resumed between the Chinese Communists and the Chinese Nationalists. While the Communists were struggling for supremacy in Manchuria, they were supported by the North Korean government with matériel and manpower. According to Chinese sources, the North Koreans donated 2,000 railway cars worth of matériel while thousands of Koreans served in the Chinese People's Liberation Army (PLA) during the war. North Korea also provided the Chinese Communists in Manchuria with a safe refuge for non-combatants and communications with the rest of China.
The North Korean contributions to the Chinese Communist victory were not forgotten after the creation of the People's Republic of China in 1949. As a token of gratitude, between 50,000 to 70,000 Korean veterans that served in the PLA were sent back along with their weapons, and they later played a significant role in the initial invasion of South Korea. China promised to support the North Koreans in the event of a war against South Korea. The Chinese support created a deep division between the Korean Communists, and Kim Il-sung's authority within the Communist party was challenged by the Chinese faction led by Pak Il-yu, who was later purged by Kim.
After the formation of the People's Republic of China in 1949, the Chinese government named the Western nations, led by the United States, as the biggest threat to its national security. Basing this judgment on China's century of humiliation beginning in the early 19th century, American support for the Nationalists during the Chinese Civil War, and the ideological struggles between revolutionaries and reactionaries, the Chinese leadership believed that China would become a critical battleground in the United States' crusade against Communism. As a countermeasure and to elevate China's standing among the worldwide Communist movements, the Chinese leadership adopted a foreign policy that actively promoted Communist revolutions throughout territories on China's periphery.
By 1949, South Korean forces had reduced the active number of communist guerrillas in the South from 5,000 to 1,000. However, Kim Il-sung believed that the guerrillas had weakened the South Korean military and that a North Korean invasion would be welcomed by much of the South Korean population. Kim began seeking Stalin's support for an invasion in March 1949, travelling to Moscow to attempt to persuade Stalin.
Initially, Stalin did not think the time was right for a war in Korea. Chinese Communist forces were still fighting in China. American forces were still stationed in South Korea (they would complete their withdrawal in June 1949) and Stalin did not want the Soviet Union to become embroiled in a war with the United States.
By spring 1950, Stalin believed the strategic situation had changed. The Soviets had detonated their first nuclear bomb in September 1949; American soldiers had fully withdrawn from Korea; the Americans had not intervened to stop the communist victory in China, and Stalin calculated that the Americans would be even less willing to fight in Korea—which had seemingly much less strategic significance. The Soviets had also cracked the codes used by the US to communicate with the US embassy in Moscow, and reading these dispatches convinced Stalin that Korea did not have the importance to the US that would warrant a nuclear confrontation.  Stalin began a more aggressive strategy in Asia based on these developments, including promising economic and military aid to China through the Sino-Soviet Friendship, Alliance, and Mutual Assistance Treaty.
Throughout 1949 and 1950 the Soviets continued to arm North Korea. After the Communist victory in the Chinese Civil War, ethnic Korean units in the Chinese People's Liberation Army (PLA) were released to North Korea. The combat veterans from China, the tanks, artillery and aircraft supplied by the Soviets, and rigorous training increased North Korea's military superiority over the South, which had been armed by the American military.
In April 1950, Stalin gave Kim permission to invade the South under the condition that Mao would agree to send reinforcements if they became needed. Stalin made it clear that Soviet forces would not openly engage in combat, to avoid a direct war with the Americans. Kim met with Mao in May 1950. Mao was concerned that the Americans would intervene but agreed to support the North Korean invasion. China desperately needed the economic and military aid promised by the Soviets. At that time, the Chinese were in the process of demobilizing half of the PLA's 5.6 million soldiers. However, Mao sent more ethnic Korean PLA veterans to Korea and promised to move an army closer to the Korean border. Once Mao's commitment was secured, preparations for war accelerated.
Soviet generals with extensive combat experience from the Second World War were sent to North Korea as the Soviet Advisory Group. These generals completed the plans for the attack by May. The original plans called for a skirmish to be initiated in the Ongjin Peninsula on the west coast of Korea. The North Koreans would then launch a "counterattack" that would capture Seoul and encircle and destroy the South Korean army. The final stage would involve destroying South Korean government remnants, capturing the rest of South Korea, including the ports.
On 7 June 1950, Kim Il-sung called for a Korea-wide election on 5–8 August 1950 and a consultative conference in Haeju on 15–17 June 1950. On 11 June, the North sent three diplomats to the South, as a peace overture that Rhee rejected. On 21 June, Kim Il-Sung revised his war plan to involve general attack across the 38th parallel, rather than a limited operation in the Ongjin peninsula. Kim was concerned that South Korean agents had learned about the plans and South Korean forces were strengthening their defenses. Stalin agreed to this change of plan.
While these preparations were underway in the North, there were frequent clashes along the 38th parallel, especially at Kaesong and Ongjin, many initiated by the South. The Republic of Korea Army (ROK Army) was being trained by the U.S. Korean Military Advisory Group (KMAG). On the eve of war, KMAG's commander General William Lynn Roberts voiced utmost confidence in the ROK Army and boasted that any North Korean invasion would merely provide "target practice". For his part, Syngman Rhee repeatedly expressed his desire to conquer the North, including when American diplomat John Foster Dulles visited Korea on 18 June.
Although some South Korean and American intelligence officers were predicting an attack from the North, similar predictions had been made before and nothing had eventuated. The Central Intelligence Agency did note the southward movement by the Korean People's Army (KPA), but assessed this as a "defensive measure" and concluded an invasion was "unlikely". On 23 June, UN observers inspected the border and did not detect that war was imminent.
At dawn on Sunday, 25 June 1950, the Korean People's Army crossed the 38th parallel behind artillery fire. The KPA justified its assault with the claim that ROK troops had attacked first, and that they were aiming to arrest and execute the "bandit traitor Syngman Rhee". Fighting began on the strategic Ongjin peninsula in the west. There were initial South Korean claims that they had captured the city of Haeju, and this sequence of events has led some scholars to argue that the South Koreans actually fired first.
Whoever fired the first shots in Ongjin, within an hour, North Korean forces attacked all along the 38th parallel. The North Koreans had a combined arms force including tanks supported by heavy artillery. The South Koreans did not have any tanks, anti-tank weapons, nor heavy artillery, that could stop such an attack. In addition, South Koreans committed their forces in a piecemeal fashion and these were routed within a few days.
On 27 June, Rhee evacuated from Seoul with some of the government. On 28 June, at 2am, the South Korean Army blew up the highway bridge across the Han River in an attempt to stop the North Korean army. The bridge was detonated while 4,000 refugees were crossing the bridge, and hundreds were killed. Destroying the bridge also trapped many South Korean military units North of the Han River. In spite of such desperate measures, Seoul fell that same day. A number of South Korean National Assemblymen remained in Seoul when it fell, and forty-eight subsequently pledged allegiance to the North.
In five days the South Korean forces, which had 95,000 men on 25 June, was down to less than 22,000 men. In early July, when U.S. forces arrived, what was left of the South Korean forces were placed under U.S. operational command of the United Nations Command.
The Truman administration was caught ill prepared and at a crossroads. Before the invasion, Korea was not included in the strategic Asian Defense Perimeter outlined by Secretary of State Dean Acheson. Military strategists were more concerned with the security of Europe against the Soviet Union than East Asia. At the same time, the Administration was worried that a war in Korea could quickly widen into another world war should the Chinese or Soviets decide to get involved as well.
One facet of the changing attitude toward Korea and whether to get involved was Japan. Especially after the fall of China to the Communists, U.S. East Asian experts saw Japan as the critical counterweight to the Soviet Union and China in the region. While there was no United States policy that dealt with South Korea directly as a national interest, its proximity to Japan increased the importance of South Korea. Said Kim: "The recognition that the security of Japan required a non-hostile Korea led directly to President Truman's decision to intervene... The essential point... is that the American response to the North Korean attack stemmed from considerations of US policy toward Japan."
A major consideration was the possible Soviet reaction in the event that the US intervened. The Truman administration was fretful that a war in Korea was a diversionary assault that would escalate to a general war in Europe once the United States committed in Korea. At the same time, "[t]here was no suggestion from anyone that the United Nations or the United States could back away from [the conflict]". Truman believed if aggression went unchecked a chain reaction would be initiated that would marginalize the United Nations and encourage Communist aggression elsewhere. The UN Security Council approved the use of force to help the South Koreans and the US immediately began using what air and naval forces that were in the area to that end. The Administration still refrained from committing on the ground because some advisers believed the North Koreans could be stopped by air and naval power alone.
The Truman administration was still uncertain if the attack was a ploy by the Soviet Union or just a test of U.S. resolve. The decision to commit ground troops became viable when a communiqué was received on 27 June indicating the Soviet Union would not move against U.S. forces in Korea. The Truman administration now believed it could intervene in Korea without undermining its commitments elsewhere.
On 25 June 1950, the United Nations Security Council unanimously condemned the North Korean invasion of the Republic of Korea, with UN Security Council Resolution 82. The Soviet Union, a veto-wielding power, had boycotted the Council meetings since January 1950, protesting that the Republic of China (Taiwan), not the People's Republic of China, held a permanent seat in the UN Security Council. After debating the matter, the Security Council, on 27 June 1950, published Resolution 83 recommending member states provide military assistance to the Republic of Korea. On 27 June President Truman ordered U.S. air and sea forces to help the South Korean regime. On 4 July the Soviet Deputy Foreign Minister accused the United States of starting armed intervention on behalf of South Korea.
The Soviet Union challenged the legitimacy of the war for several reasons. The ROK Army intelligence upon which Resolution 83 was based came from U.S. Intelligence; North Korea was not invited as a sitting temporary member of the UN, which violated UN Charter Article 32; and the Korean conflict was beyond the UN Charter's scope, because the initial north–south border fighting was classed as a civil war. Because the Soviet Union was boycotting the Security Council at the time, legal scholars posited that deciding upon an action of this type required the unanimous vote of the five permanent members.
By mid-1950, North Korean forces numbered between 150,000 and 200,000 troops, organized into 10 infantry divisions, one tank division, and one air force division, with 210 fighter planes and 280 tanks, who captured scheduled objectives and territory, among them Kaesong, Chuncheon, Uijeongbu, and Ongjin. Their forces included 274 T-34-85 tanks, some 150 Yak fighters, 110 attack bombers, 200 artillery pieces, 78 Yak trainers, and 35 reconnaissance aircraft. In addition to the invasion force, the North KPA had 114 fighters, 78 bombers, 105 T-34-85 tanks, and some 30,000 soldiers stationed in reserve in North Korea. Although each navy consisted of only several small warships, the North and South Korean navies fought in the war as sea-borne artillery for their in-country armies.
In contrast, the ROK Army defenders were relatively unprepared and ill-equipped. In South to the Naktong, North to the Yalu (1961), R.E. Appleman reports the ROK forces' low combat readiness as of 25 June 1950. The ROK Army had 98,000 soldiers (65,000 combat, 33,000 support), no tanks (they had been requested from the U.S. military, but requests were denied), and a 22-piece air force comprising 12 liaison-type and 10 AT6 advanced-trainer airplanes. There were no large foreign military garrisons in Korea at the time of the invasion, but there were large U.S. garrisons and air forces in Japan.
On Saturday, 24 June 1950, U.S. Secretary of State Dean Acheson informed President Truman that the North Koreans had invaded South Korea. Truman and Acheson discussed a U.S. invasion response and agreed that the United States was obligated to act, paralleling the North Korean invasion with Adolf Hitler's aggressions in the 1930s, with the conclusion being that the mistake of appeasement must not be repeated. Several U.S. industries were mobilized to supply materials, labor, capital, production facilities, and other services necessary to support the military objectives of the Korean War. However, President Truman later acknowledged that he believed fighting the invasion was essential to the American goal of the global containment of communism as outlined in the National Security Council Report 68 (NSC-68) (declassified in 1975):
Communism was acting in Korea, just as Hitler, Mussolini and the Japanese had ten, fifteen, and twenty years earlier. I felt certain that if South Korea was allowed to fall, Communist leaders would be emboldened to override nations closer to our own shores. If the Communists were permitted to force their way into the Republic of Korea without opposition from the free world, no small nation would have the courage to resist threat and aggression by stronger Communist neighbors.
In August 1950, the President and the Secretary of State obtained the consent of Congress to appropriate $12 billion for military action in Korea.
As an initial response, Truman called for a naval blockade of North Korea, and was shocked to learn that such a blockade could only be imposed 'on paper', since the U.S. Navy no longer had the warships with which to carry out his request. In fact, because of the extensive defense cuts and the emphasis placed on building a nuclear bomber force, none of the services were in a position to make a robust response with conventional military strength. General Omar Bradley, Chairman of the Joint Chiefs of Staff, was faced with re-organizing and deploying an American military force that was a shadow of its World War II counterpart. The impact of the Truman administration's defense budget cutbacks were now keenly felt, as American troops fought a series of costly rearguard actions. Lacking sufficient anti-tank weapons, artillery or armor, they were driven back down the Korean peninsula to Pusan. In a postwar analysis of the unpreparedness of U.S. Army forces deployed to Korea during the summer and fall of 1950, Army Major General Floyd L. Parks stated that "Many who never lived to tell the tale had to fight the full range of ground warfare from offensive to delaying action, unit by unit, man by man ... [T]hat we were able to snatch victory from the jaws of defeat ... does not relieve us from the blame of having placed our own flesh and blood in such a predicament."
Acting on State Secretary Acheson's recommendation, President Truman ordered General MacArthur to transfer matériel to the Army of the Republic of Korea while giving air cover to the evacuation of U.S. nationals. The President disagreed with advisers who recommended unilateral U.S. bombing of the North Korean forces, and ordered the US Seventh Fleet to protect the Republic of China (Taiwan), whose government asked to fight in Korea. The United States denied ROC's request for combat, lest it provoke a communist Chinese retaliation. Because the United States had sent the Seventh Fleet to "neutralize" the Taiwan Strait, Chinese premier Zhou Enlai criticized both the UN and U.S. initiatives as "armed aggression on Chinese territory."
The Battle of Osan, the first significant American engagement of the Korean War, involved the 540-soldier Task Force Smith, which was a small forward element of the 24th Infantry Division which had been flown in from Japan. On 5 July 1950, Task Force Smith attacked the North Koreans at Osan but without weapons capable of destroying the North Koreans' tanks. They were unsuccessful; the result was 180 dead, wounded, or taken prisoner. The KPA progressed southwards, pushing back the US force at Pyongtaek, Chonan, and Chochiwon, forcing the 24th Division's retreat to Taejeon, which the KPA captured in the Battle of Taejon; the 24th Division suffered 3,602 dead and wounded and 2,962 captured, including the Division's Commander, Major General William F. Dean.
By August, the KPA had pushed back the ROK Army and the Eighth United States Army to the vicinity of Pusan in southeast Korea. In their southward advance, the KPA purged the Republic of Korea's intelligentsia by killing civil servants and intellectuals. On 20 August, General MacArthur warned North Korean leader Kim Il-sung that he was responsible for the KPA's atrocities. By September, the UN Command controlled the Pusan perimeter, enclosing about 10% of Korea, in a line partially defined by the Nakdong River.
Although Kim's early successes had led him to predict that he would end the war by the end of August, Chinese leaders were more pessimistic. To counter a possible U.S. deployment, Zhou Enlai secured a Soviet commitment to have the Soviet Union support Chinese forces with air cover, and deployed 260,000 soldiers along the Korean border, under the command of Gao Gang. Zhou commanded Chai Chengwen to conduct a topographical survey of Korea, and directed Lei Yingfu, Zhou's military advisor in Korea, to analyze the military situation in Korea. Lei concluded that MacArthur would most likely attempt a landing at Incheon. After conferring with Mao that this would be MacArthur's most likely strategy, Zhou briefed Soviet and North Korean advisers of Lei's findings, and issued orders to Chinese army commanders deployed on the Korean border to prepare for American naval activity in the Korea Strait.
In the resulting Battle of Pusan Perimeter (August–September 1950), the U.S. Army withstood KPA attacks meant to capture the city at the Naktong Bulge, P'ohang-dong, and Taegu. The United States Air Force (USAF) interrupted KPA logistics with 40 daily ground support sorties that destroyed 32 bridges, halting most daytime road and rail traffic. KPA forces were forced to hide in tunnels by day and move only at night. To deny matériel to the KPA, the USAF destroyed logistics depots, petroleum refineries, and harbors, while the U.S. Navy air forces attacked transport hubs. Consequently, the over-extended KPA could not be supplied throughout the south.
Meanwhile, U.S. garrisons in Japan continually dispatched soldiers and matériel to reinforce defenders in the Pusan Perimeter. Tank battalions deployed to Korea directly from the U.S. mainland from the port of San Francisco to the port of Pusan, the largest Korean port. By late August, the Pusan Perimeter had some 500 medium tanks battle-ready. In early September 1950, ROK Army and UN Command forces outnumbered the KPA 180,000 to 100,000 soldiers. The UN forces, once prepared, counterattacked and broke out of the Pusan Perimeter.
Against the rested and re-armed Pusan Perimeter defenders and their reinforcements, the KPA were undermanned and poorly supplied; unlike the UN Command, they lacked naval and air support. To