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The Korean War 101: Causes, Course, and Conclusion of the Conflict

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people taking photos of a distant valley
two South Korean Army officers observing activities in Communist territory just across the thirty-eighth parallel. this picture was made just at the outbreak of the Korean War. Source: the uS Army Korean War Flickr page at

North Korea attacked South Korea on June 25, 1950, igniting the Korean War. Cold War assumptions governed the immediate reaction of US leaders, who instantly concluded that Soviet Premier Joseph Stalin had ordered the invasion as the first step in his plan for world conquest. “Communism,” President Harry S. Truman argued later in his memoirs, “was acting in Korea just as [Adolf] Hitler, [Benito] Mussolini, and the Japanese had acted ten, fifteen, and twenty years earlier.” If North Korea’s aggression went “unchallenged, the world was certain to be plunged into another world war.” This 1930s history lesson prevented Truman from recognizing that the origins of this conflict dated to at least the start of World War II, when Korea was a colony of Japan. Liberation in August 1945 led to division and a predictable war because the US and the Soviet Union would not allow the Korean people to decide their own future.

Before 1941, the US had no vital interests in Korea and was largely indifferent to its fate.

photo of three men sitting together
Prime Minister Winston Churchill, President Roosevelt, and Stalin at the yalta Conference,
February 1945. Source:

Before 1941, the US had no vital interests in Korea and was largely in- different to its fate. But after Pearl Harbor, President Franklin D. Roosevelt and his advisors acknowledged at once the importance of this strategic peninsula for peace in Asia, advocating a postwar trusteeship to achieve Korea’s independence. Late in 1943, Roosevelt joined British Prime Minister Winston Churchill and Chinese Generalissimo Chiang Kaishek in signing the Cairo Declaration, stating that the Allies “are determined that in due course Korea shall become free and independent.” At the Yalta Conference in early 1945, Stalin endorsed a four-power trusteeship in Korea. When Harry S. Truman became president after Roosevelt’s death in April 1945, however, Soviet expansion in Eastern Europe had begun to alarm US leaders. An atomic attack on Japan, Truman thought, would preempt Soviet entry into the Pacific War and allow unilateral American occupation of Korea. His gamble failed. On August 8, Stalin declared war on Japan and sent the Red Army into Korea. Only Stalin’s acceptance of Truman’s eleventh-hour proposal to divide the peninsula into So- viet and American zones of military occupation at the thirty-eighth parallel saved Korea from unification under Communist rule.

Deterioration of Soviet-American relations in Europe meant that neither side was willing to acquiesce in any agreement in Korea that might strengthen its adversary.

a photo of several men in uniform
Kim il-sung (R) with visiting Soviet military officials in Pyongyang, 1947. Source: Korea Times at

US military occupation of southern Korea began on September 8, 1945. With very little preparation, Washing- ton redeployed the XXIV Corps under the command of Lieutenant General John R. Hodge from Okinawa to Korea. US occupation officials, ignorant of Korea’s history and culture, quickly had trouble maintaining order because al- most all Koreans wanted immediate in- dependence. It did not help that they followed the Japanese model in establishing an authoritarian US military government. Also, American occupation officials relied on wealthy land- lords and businessmen who could speak English for advice. Many of these citizens were former Japanese collaborators and had little interest in ordinary Koreans’ reform demands. Meanwhile, Soviet military forces in northern Korea, after initial acts of rape, looting, and petty crime, implemented policies to win popular support. Working with local people’s committees and indigenous Communists, Soviet officials enacted sweeping political, social, and economic changes. They also expropriated and punished landlords and collaborators, who fled southward and added to rising distress in the US zone. Simultaneously, the Soviets ignored US requests to coordinate occupation policies and allow free traffic across the parallel.

a group photo of men in military uniforms
South Korea’s President Syngman Rhee (R), general Douglas MacArthur (C), and Lieutenant general John R. Hodge (L) attending ceremonies on August 15, 1948, to mark the founding of the Republic of Korea. Source: Korea Times.

Deterioration of Soviet-American relations in Europe meant that neither side was willing to acquiesce in any agreement in Korea that might strengthen its adversary. This became clear when the US and the Soviet Union tried to implement a revived trusteeship plan after the Moscow Conference in December 1945. Eighteen months of intermittent bilateral negotiations in Korea failed to reach agreement on a representative group of Koreans to form a provisional government, primarily because Moscow refused to consult with anti-Communist politicians opposed to trustee- ship. Meanwhile, political instability and economic deterioration in southern Korea persisted, causing Hodge to urge withdrawal. Postwar US demobilization that brought steady reductions in defense spending fueled pressure for disengagement. In September 1947, the Joint Chiefs of Staff (JCS) added weight to the withdrawal argument when they advised that Korea held no strategic significance. With Communist power growing in China, however, the Truman administration was unwilling to abandon southern Korea precipitously, fearing domestic criticism from Republicans and damage to US credibility abroad.

Seeking an answer to its dilemma, the US referred the Korean dispute to the United Nations, which passed a resolution late in 1947 calling for internationally supervised elections for a government to rule a united Korea. Truman and his advisors knew the Soviets would refuse to cooper- ate. Discarding all hope for early reunification, US policy by then had shifted to creating a separate South Korea, able to defend itself. Bowing to US pressure, the United Nations supervised and certified as valid obviously undemocratic elections in the south alone in May 1948, which resulted in formation of the Republic of Korea (ROK) in August. The Soviet Union responded in kind, sponsoring the creation of the Democratic People’s Re- public of Korea (DPRK) in September. There now were two Koreas, with President Syngman Rhee installing a repressive, dictatorial, and anti-Communist regime in the south, while wartime guerrilla leader Kim Il Sung imposed the totalitarian Stalinist model for political, economic, and social development on the north. A UN resolution then called for Soviet-American withdrawal. In December 1948, the Soviet Union, in response to the DPRK’s request, removed its forces from North Korea.

South Korea’s new government immediately faced violent opposition, climaxing in October 1948 with the Yosu-Sunchon Rebellion. Despite plans to leave the south by the end of 1948, Truman delayed military withdrawal until June 29, 1949. By then, he had approved National Security Council (NSC) Paper 8/2, undertaking a commitment to train, equip, and supply an ROK security force capable of maintaining internal order and deterring a DPRK attack. In spring 1949, US military advisors supervised a dramatic improvement in ROK army fighting abilities. They were so successful that militant South Korean officers began to initiate assaults northward across the thirty-eighth parallel that summer. These attacks ignited major border clashes with North Korean forces. A kind of war was already underway on the peninsula when the conventional phase of Korea’s conflict began on June 25, 1950. Fears that Rhee might initiate an offensive to achieve reunification explain why the Truman administration limited ROK military capabilities, withholding tanks, heavy artillery, and warplanes.

photo of two men in military uniforms
Kim il Sung in China with Mao, early 1950s. Source: Screen capture from a North Korean video, “Kim ilsung de-wons man-manse,” on youtube at

Pursuing qualified containment in Korea, Truman asked Congress for three-year funding of economic aid to the ROK in June 1949. To build sup- port for its approval, on January 12, 1950, Secretary of State Dean G. Ache- son’s speech to the National Press Club depicted an optimistic future for South Korea. Six months later, critics charged that his exclusion of the ROK from the US “defensive perimeter” gave the Communists a “green light” to launch an invasion. However, Soviet documents have established that Acheson’s words had almost no impact on Communist invasion planning. Moreover, by June 1950, the US policy of containment in Korea through economic means appeared to be experiencing marked success. The ROK had acted vigorously to control spiraling inflation, and Rhee’s opponents won legislative control in May elections. As important, the ROK army virtually eliminated guerrilla activities, threatening internal order in South Korea, causing the Truman administration to propose a sizeable military aid increase. Now optimistic about the ROK’s prospects for survival, Washington wanted to deter a conventional attack from the north.

Stalin worried about South Korea’s threat to North Korea’s survival. Throughout 1949, he consistently refused to approve Kim Il Sung’s persistent requests to authorize an attack on the ROK. Communist victory in China in fall 1949 pressured Stalin to show his support for a similar Korean outcome. In January 1950, he and Kim discussed plans for an invasion in Moscow, but the Soviet dictator was not ready to give final consent. How- ever, he did authorize a major expansion of the DPRK’s military capabilities. At an April meeting, Kim Il Sung persuaded Stalin that a military victory would be quick and easy because of southern guerilla support and an anticipated popular uprising against Rhee’s regime. Still fearing US military intervention, Stalin informed Kim that he could invade only if Mao Zedong approved. During May, Kim Il Sung went to Beijing to gain the consent of the People’s Republic of China (PRC). Significantly, Mao also voiced concern that the Americans would defend the ROK but gave his reluctant approval as well. Kim Il Sung’s patrons had joined in approving his reckless decision for war.

a man in a suit holds his hand up in greeting
President Harry S. truman getting ready to address the nation on the Korean situation from the White House on July 19, 1950. Source: the truman Library at An audio file of his address to the nation is available at

On the morning of June 25, 1950, the Korean People’s Army (KPA) launched its military offensive to conquer South Korea. Rather than immediately committing ground troops, Truman’s first action was to approve referral of the matter to the UN Security Council because he hoped the ROK military could defend itself with primarily indirect US assistance. The UN Security Council’s first resolution called on North Korea to accept a cease- fire and withdraw, but the KPA continued its advance. On June 27, a second resolution requested that member nations provide support for the ROK’s defense. Two days later, Truman, still optimistic that a total commitment was avoidable, agreed in a press conference with a newsman’s description of the conflict as a “police action.” His actions reflected an existing policy that sought to block Communist expansion in Asia without using US military power, thereby avoiding increases in defense spending. But early on June 30, he reluctantly sent US ground troops to Korea after General Douglas MacArthur, US Occupation commander in Japan, advised that failure to do so meant certain Communist destruction of the ROK.

Kim Il Sung’s patrons [Stalin and Mao] had joined in approving his reckless decision for war.

On July 7, 1950, the UN Security Council created the United Nations Command (UNC) and called on Truman to appoint a UNC commander. The president immediately named MacArthur, who was required to submit periodic reports to the United Nations on war developments. The ad- ministration blocked formation of a UN committee that would have direct access to the UNC commander, instead adopting a procedure whereby MacArthur received instructions from and reported to the JCS. Fifteen members joined the US in defending the ROK, but 90 percent of forces were South Korean and American with the US providing weapons, equipment, and logistical support. Despite these American commitments, UNC forces initially suffered a string of defeats. By July 20, the KPA shattered five US battalions as it advanced one hundred miles south of Seoul, the ROK capital. Soon, UNC forces finally stopped the KPA at the Pusan Perimeter, a rectangular area in the southeast corner of the peninsula.

On September 11, 1950, Truman had approved NSC-81, a plan to cross the thirty-eighth parallel and forcibly reunify Korea

Despite the UNC’s desperate situation during July, MacArthur developed plans for a counteroffensive in coordination with an amphibious landing behind enemy lines allowing him to “compose and unite” Korea. State Department officials began to lobby for forcible reunification once the UNC assumed the offensive, arguing that the US should destroy the KPA and hold free elections for a government to rule a united Korea. The JCS had grave doubts about the wisdom of landing at the port of Inchon, twenty miles west of Seoul, because of narrow access, high tides, and sea- walls, but the September 15 operation was a spectacular success. It allowed the US Eighth Army to break out of the Pusan Perimeter and advance north to unite with the X Corps, liberating Seoul two weeks later and sending the KPA scurrying back into North Korea. A month earlier, the administration had abandoned its initial war aim of merely restoring the status quo. On September 11, 1950, Truman had approved NSC-81, a plan to cross the thirty-eighth parallel and forcibly reunify Korea.

Invading the DPRK was an incredible blunder that transformed a three-month war into one lasting three years. US leaders had realized that extension of hostilities risked Soviet or Chinese entry, and therefore, NSC- 81 included the precaution that only Korean units would move into the most northern provinces. On October 2, PRC Foreign Minister Zhou Enlai warned the Indian ambassador that China would intervene in Korea if US forces crossed the parallel, but US officials thought he was bluffing. The UNC offensive began on October 7, after UN passage of a resolution authorizing MacArthur to “ensure conditions of stability throughout Korea.” At a meeting at Wake Island on October 15, MacArthur assured Truman that China would not enter the war, but Mao already had decided to intervene after concluding that Beijing could not tolerate US challenges to its regional credibility. He also wanted to repay the DPRK for sending thou- sands of soldiers to fight in the Chinese civil war. On August 5, Mao instructed his northeastern military district commander to prepare for operations in Korea in the first ten days of September. China’s dictator then muted those associates opposing intervention.

men in military uniforms
Brigadier general Courtney Whitney (L), general Douglas MacArthur, Commander in Chief of
uN Forces (C); and Major general edward M. Almond (R) observe the shelling of inchon from
the USS Mount McKinley, September 15, 1950. Source: the Army Signal Corps Collection in the US National Archives at

On October 19, units of the Chinese People’s Volunteers (CPV) under the command of General Peng Dehuai crossed the Yalu River. Five days later, MacArthur ordered an offensive to China’s border with US forces in the vanguard. When the JCS questioned this violation of NSC-81, MacArthur replied that he had discussed this action with Truman on Wake Island. Having been wrong in doubting Inchon, the JCS remained silent this time. Nor did MacArthur’s superiors object when he chose to retain a divided command. Even after the first clash between UNC and CPV troops on October 26, the general remained supremely confident. One week later, the Chinese sharply attacked advancing UNC and ROK forces. In response, MacArthur ordered air strikes on Yalu bridges without seeking Washing- ton’s approval. Upon learning this, the JCS prohibited the assaults, pending Truman’s approval. MacArthur then asked that US pilots receive permission for “hot pursuit” of enemy aircraft fleeing into Manchuria. He was infuriated upon learning that the British were advancing a UN proposal to halt the UNC offensive well short of the Yalu to avert war with China, viewing the measure as appeasement.

photo of two men in uniforms
general MacArthur and President truman at Wake island, October, 15, 1950.
Source: the truman Library at

On November 24, MacArthur launched his “Home-by-Christmas Offensive.” The next day, the CPV counterattacked en masse, sending UNC forces into a chaotic retreat southward and causing the Truman administration immediately to consider pursuing a Korean cease-fire. In several public pronouncements, MacArthur blamed setbacks not on himself but on unwise command limitations. In response, Truman approved a directive to US officials that State Department approval was required for any comments about the war. Later that month, MacArthur submitted a four- step “Plan for Victory” to defeat the Communists—a naval blockade of China’s coast, authorization to bombard military installations in Manchuria, deployment of Chiang Kai-shek Nationalist forces in Korea, and launching of an attack on mainland China from Taiwan. The JCS, despite later denials, considered implementing these actions before receiving favorable battlefield reports.

Early in 1951, Lieutenant General Matthew B. Ridgway, new commander of the US Eighth Army, halted the Communist southern advance. Soon, UNC counterattacks restored battle lines north of the thirty-eighth parallel. In March, MacArthur, frustrated by Washington’s refusal to escalate the war, issued a demand for immediate surrender to the Communists that sabotaged a planned cease-fire initiative. Truman reprimanded but did not recall the general. On April 5, House Republican Minority Leader Joseph W. Martin Jr. read MacArthur’s letter in Congress, once again criticizing the administration’s efforts to limit the war. Truman later argued that this was the “last straw.” On April 11, with the unanimous support of top advisors, the president fired MacArthur, justifying his action as a defense of the constitutional principle of civilian control over the military, but another consideration may have exerted even greater influence on Truman. The JCS had been monitoring a Communist military buildup in East Asia and thought a trusted UNC commander should have standing authority to retaliate against Soviet or Chinese escalation, including the use of nuclear weapons that they had deployed to forward Pacific bases. Truman and his advisors, as well as US allies, distrusted MacArthur, fearing that he might provoke an incident to widen the war.

MacArthur’s recall ignited a firestorm of public criticism against both Truman and the war. The general returned to tickertape parades and, on April 19, 1951, he delivered a televised address before a joint session of Congress, defending his actions and making this now-famous assertion: “In war there is no substitute for victory.” During Senate joint committee hearings on his firing in May, MacArthur denied that he was guilty of in- subordination. General Omar N. Bradley, the JCS chair, made the administration’s case, arguing that enacting MacArthur’s proposals would lead to “the wrong war, at the wrong place, at the wrong time, and with the wrong enemy.” Meanwhile, in April, the Communists launched the first of two major offensives in a final effort to force the UNC off the peninsula. When May ended, the CPV and KPA had suffered huge losses, and a UNC counteroffensive then restored the front north of the parallel, persuading Beijing and Pyongyang, as was already the case in Washington, that pursuit of a cease-fire was necessary. The belligerents agreed to open truce negotiations on July 10 at Kaesong, a neutral site that the Communists deceitfully occupied on the eve of the first session.

North Korea and China created an acrimonious atmosphere with at- tempts at the outset to score propaganda points, but the UNC raised the first major roadblock with its proposal for a demilitarized zone extending deep into North Korea. More important, after the talks moved to Panmunjom in October, there was rapid progress in resolving almost all is- sues, including establishment of a demilitarized zone along the battle lines, truce enforcement inspection procedures, and a postwar political conference to discuss withdrawal of foreign troops and reunification. An armistice could have been concluded ten months after talks began had the negotiators not deadlocked over the disposition of prisoners of war (POWs). Rejecting the UNC proposal for non-forcible repatriation, the Communists demanded adherence to the Geneva Convention that required return of all POWs. Beijing and Pyongyang were guilty of hypocrisy regarding this matter because they were subjecting UNC prisoners to unspeakable mistreatment and indoctrination.

On April 11, with the unanimous support of top advisors, the presi- dent fired MacArthur.

a man holds newspapers and yells
the news that truman fired general MacArthur hits the streets. Source: the website of the Authentic History Center: Primary Sources from American Popular Culture at

Truman ordered that the UNC delegation assume an inflexible stand against returning Communist prisoners to China and North Korea against their will. “We will not buy an armistice,” he insisted, “by turning over human beings for slaughter or slavery.” Although Truman unquestionably believed in the moral rightness of his position, he was not unaware of the propaganda value derived from Communist prisoners defecting to the “free world.” His advisors, however, withheld evidence from him that contradicted this assessment. A vast majority of North Korean POWs were actually South Koreans who either joined voluntarily or were impressed into the KPA. Thousands of Chinese POWs were Nationalist soldiers trapped in China at the end of the civil war, who now had the chance to escape to Taiwan. Chinese Nationalist guards at UNC POW camps used terrorist “re-education” tactics to compel prisoners to refuse repatriation; resisters risked beatings or death, and repatriates were even tattooed with anti- Communist slogans.

In November 1952, angry Americans elected Dwight D. Eisenhower president, in large part because they expected him to end what had be- come the very unpopular “Mr. Truman’s War.” Fulfilling a campaign pledge, the former general visited Korea early in December, concluding that further ground attacks would be futile. Simultaneously, the UN General Assembly called for a neutral commission to resolve the dispute over POW repatriation. Instead of embracing the plan, Eisenhower, after taking office in January 1953, seriously considered threatening a nuclear attack on China to force a settlement. Signaling his new resolve, Eisenhower announced on February 2 that he was ordering removal of the US Seventh Fleet from the Taiwan Strait, implying endorsement for a Nationalist assault on the mainland. What influenced China more was the devastating impact of the war. By summer 1952, the PRC faced huge domestic economic problems and likely decided to make peace once Truman left office. Major food shortages and physical devastation persuaded Pyongyang to favor an armistice even earlier.

An armistice ended fighting in Korea on July 27, 1953.

men in military uniforms and signing documents
the Chinese general, Peng Dehuai, signs the 1953 Korean Armistice Agreement.

Early in 1953, China and North Korea were prepared to resume the truce negotiations, but the Communists preferred that the Americans make the first move. That came on February 22 when the UNC, repeating a Red Cross proposal, suggested exchanging sick and wounded prisoners. At this key moment, Stalin died on March 5. Rather than dissuading the PRC and the DPRK as Stalin had done, his successors encouraged them to act on their desire for peace. On March 28, the Communist side accepted the UNC proposal. Two days later, Zhou Enlai publicly proposed transfer of prisoners rejecting repatriation to a neutral state. On April 20, Operation Little Switch, the exchange of sick and wounded prisoners, began, and six days later, negotiations resumed at Panmunjom. Sharp disagreement followed over the final details of the truce agreement. Eisenhower insisted later that the PRC accepted US terms after Secretary of State John Foster Dulles informed India’s prime minister in May that without progress toward a truce, the US would terminate the existing limitations on its conduct of the war. No documentary evidence has of yet surfaced to support his assertion.

photo of men in military uniforms signing a document
Premier Kim il Sung endorses the July 27, 1953, Korean Armistice Agreement (Courtesy, eastphoto.)

Also, by early 1953, both Washington and Beijing clearly wanted an armistice, having tired of the economic burdens, military losses, political and military constraints, worries about an expanded war, and pressure from allies and the world community to end the stalemated conflict. A steady stream of wartime issues threatened to inflict irrevocable damage on US relations with its allies in Western Europe and nonaligned members of the United Nations. Indeed, in May 1953, US bombing of North Korea’s dams and irrigation system ignited an outburst of world criticism. Later that month and early in June, the CPV staged powerful attacks against ROK defensive positions. Far from being intimidated, Beijing thus displayed its continuing resolve, using military means to persuade its adversary to make concessions on the final terms. Before the belligerents could sign the agreement, Rhee tried to torpedo the impending truce when he released 27,000 North Korean POWs. Eisenhower bought Rhee’s acceptance of a cease-fire with pledges of financial aid and a mutual security pact.

An armistice ended fighting in Korea on July 27, 1953. Since then, Koreans have seen the war as the second-greatest tragedy in their recent history after Japanese colonial rule. Not only did it cause devastation and three million deaths, it also confirmed the division of a homogeneous society after thirteen centuries of unity, while permanently separating millions of families. Meanwhile, US wartime spending jump-started Japan’s economy, which led to its emergence as a global power. Koreans instead had to endure the living tragedy of yearning for reunification, as diplomatic tension and military clashes along the demilitarized zone continued into the twenty-first century.

Korea’s war also dramatically reshaped world affairs. In response, US leaders vastly increased defense spending, strengthened the North Atlantic Treaty Organization militarily, and pressed for rearming West Germany. In Asia, the conflict saved Chiang’s regime on Taiwan, while making South Korea a long-term client of the US. US relations with China were poisoned for twenty years, especially after Washington persuaded the United Nations to condemn the PRC for aggression in Korea. Ironically, the war helped Mao’s regime consolidate its control in China, while elevating its regional prestige. In response, US leaders, acting on what they saw as Korea’s primary lesson, relied on military means to meet the challenge, with disastrous results in Việt Nam.


Kaufman, Burton I. The Korean Conflict. Westport, CT: Greenwood, 1999.

“Korea: Lessons of the Forgotten War.” YouTube video, 2:20, posted by KRT Productions Inc., 2000.

Lee, Steven Hugh. The Korean War. New York: Longman, 2001.

Matray, James I. “Korea’s War at Sixty: A Survey of the Literature.” Cold War History 11, no. 1 (February 2011): 99–129.

US Department of Defense. Korea 1950–1953, accessed July 9, 2012,