Korean War  conflict that began in June 1950 between the Democratic People's Republic of Korea (North Korea) and the Republic of Korea (South Korea), in which an estimated 3,000,000 persons lost their lives. The United Nations, with the United States as the principal participant, joined the war on the side of the South Koreans, and the People's Republic of China eventually came to North Korea's aid. After exceptional vicissitudes, the war was ended inconclusively in July 1953; it established a precedent for United States intervention to contain Communist expansion.

At the end of World War II, the Allies agreed that Soviet forces would accept the surrender of Japanese troops in Korea north of the 38th degree of latitude, while American troops would accept the Japanese surrender south of that line. In 1947, after the failure of negotiations to achieve the unification of the two separate Korean states that had thus been created, the United States turned the problem over to the United Nations. The Soviet Union refused to cooperate with UN plans to hold general elections in the two Koreas, and as a result, a Communist state was permanently established under Soviet auspices in the north and a pro-Western state was set up in the south. By 1949 both the United States and the Soviet Union had withdrawn the majority of their troops from the Korean Peninsula.

On June 25, 1950, the North Koreans, with the tacit approval of the Soviet Union, unleashed a carefully planned attack southward across the 38th parallel. The United Nations Security Council met in emergency session and passed a resolution calling for the assistance of all UN members in halting the North Korean invasion. (The Soviet delegate, who was absent from the Security Council in protest against the UN's failure to admit the People's Republic of China, was not present to veto the council's decision.) On June 27, U.S. president Harry S. Truman, without asking Congress to declare war, ordered United States forces to come to the assistance of South Korea as part of the UN "police action."

Meanwhile, the South Korean army was overwhelmed by the North Korean forces, and the four ill-equipped American divisions that had been rushed into the battle were driven all the way southward across the Korean Peninsula to a small area covering the approaches to Pusan, on the peninsula's southeastern tip. The American forces there were heavily reinforced, however, and then on September 15, troops commanded by General Douglas MacArthur made a daring amphibious landing at Inch'on (see photograph), about 100 miles (160 km) below the 38th parallel and on a line with Seoul, the South Korean capital. This brilliant landing far north of the main battlefront succeeded in cutting the North Korean forces' lines; the North Korean army was then totally shattered by the convergence of Allied forces from north and south, and more than 125,000 prisoners were captured by the Allies.

As the Allied forces now advanced northward back to the 38th parallel, the Chinese warned that the presence of UN forces in North Korea would be unacceptable to the security of the Chinese People's Republic and would force the Chinese to intervene in the war. UN forces, however, ignored the warnings and advanced into North Korea with the expressed intention of unifying the country. By mid-November the Allied forces were nearing the Yalu River, which marked the border between North Korea and Manchuria, the northeast part of China. The Chinese considered the approach of UN forces to the Yalu to be an unacceptable threat to Manchuria. On November 24 MacArthur announced his "Home by Christmas" offensive, in which his forces would boldly advance right up to the Yalu. The next day approximately 180,000 Chinese "volunteers" entered the war, and by December 15, after bitter winter fighting and a harrowing retreat, the Allied troops had been driven southward back to the 38th parallel. On Dec. 31, 1950, the Communists began their second invasion of South Korea with about 500,000 troops, but their attack soon faltered in the face of incessant Allied aerial bombing campaigns, and the front lines eventually stabilized along the 38th parallel.

Meanwhile, MacArthur was demanding the authority to blockade China's coastline and bomb its Manchurian bases. Truman refused, feeling that such a course would bring the Soviet Union into the war and thus lead to a global conflict. In response, MacArthur appealed over Truman's head directly to the American public in an effort to enlist support for his war aims. On April 11, 1951, President Truman relieved MacArthur as UN commander and as commander of U.S. forces in the Far East and replaced him with General Matthew B. Ridgway. On July 10, 1951, truce talks began while the North Koreans and Chinese vainly strove for further success on the battlefield. The negotiations dragged on for months, until after the U.S. presidential elections in the fall of 1952 and the victory of Dwight D. Eisenhower, who had criticized the unpopular war and announced his intention to visit Korea if elected. Eisenhower secretly informed the North Koreans and Chinese that he was prepared to use nuclear weapons and would also carry the war to China if a peace agreement was not reached. After a brief renewal of hostilities in June 1953, an armistice was concluded on July 27, and the front line was accepted as the de facto boundary between North and South Korea. The exchange and repatriation of prisoners soon followed.

The Korean War resulted in the deaths of about 1,300,000 South Koreans, many of whom were civilians; 1,000,000 Chinese; 500,000 North Koreans; and about 54,000 Americans, with much smaller numbers of British, Australian, and Turkish casualties on the Allied side. Several million Koreans temporarily became refugees, and much of South Korea's industrial plant was damaged, while North Korea was utterly devastated by American bombing campaigns.

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In early 1949, North Korea seemed to be on a war footing. Kim's New Year's speech was bellicose and excoriated South Korea as a puppet state. The army expanded rapidly, soldiers drilled in war maneuvers, and bond drives began to amass the necessary funds to purchase Soviet weaponry. The thirty-eighth parallel was fortified, and border incidents began breaking out. Neither Seoul nor P'yongyang recognized the parallel as a permanent legitimate boundary.

Although many aspects of the Korean War remained murky, it seemed that the beginning of conventional war in June 1950 was mainly Kim's decision, and that the key enabling factor was the existence of as many as 100,000 troops with battle experience in China. When the Rhee regime, with help from United States military advisers, severely reduced the guerrilla threat in the winter of 1949-50, the civil war moved into a conventional phase. Kim sought Stalin's backing for his assault, but documents from Soviet and Chinese sources suggested that he got more support from China.

Beginning on June 25, 1950, North Korean forces fought their way south through Seoul. South Korean resistance collapsed as the roads south of Seoul became blocked with refugees, who were fleeing North Korean columns spearheaded with tanks supplied by the Soviet Union. Task Force Smith, the first United States troops to enter the war, made a futile stand at Suwn, a town some thirty miles south of Seoul. Within a month of the start of the invasion, North Korean forces had seized all but a small corner of southeastern Korea anchored by the port city of Pusan. Repeated North Korean efforts, blunted by heavy United States Air Force bombing and stubborn resistance by the combined United States and South Korean forces on the Pusan perimeter, denied Kim Il Sung forceful reunification of the peninsula. The fortunes of war reversed abruptly in early September when General MacArthur boldly landed his forces at Inch'n, the port city for Seoul in west central Korea. This action severed the lines of communication and supply between the North Korean army and its base in the north. The army quickly collapsed, and combined United States and South Korean forces drove Kim Il Sung's units northward and into complete defeat.

The United States thrust in the fall of 1950, however, motivated China to bring its forces--the Chinese People's Volunteer Army--in on the northern side; these "volunteers" and the North Korean army pushed United States and South Korean forces out of North Korea within a month. Although the war lasted another two years, until the summer of 1953, the outcome of early 1951 was definitive: both a stalemate and a United States commitment to containment that accepted the de facto reality of two Koreas.

By the time the armistice was signed in 1953, North Korea had been devastated by three years of bombing attacks that had left almost no modern buildings standing. Both Koreas had watched as their country was ravaged and the expectations of 1945 were turned into a nightmare. Furthermore, when Kim's regime was nearly extinguished in the fall of 1950, the Soviet Union did very little to save it--China picked up the pieces.

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In the meantime, the communists had built a formidable political and military structure in North Korea under the aegis of the Soviet command. They had created a regional Five-Province Administrative Bureau in October 1945, which was reorganized into the North Korean Provisional People's Committee in February 1946 and shed the "Provisional" component of its name twelve months later. The communists also expanded and consolidated their party's strength by merging all of the left-wing groups into the North Korean Workers' Party in August 1946. Beginning in 1946, the armed forces also were organized and reinforced. Between 1946 and 1949, large numbers of North Korean youths--at least 10,000--were taken to the Soviet Union for military training. A draft was instituted, and in 1949 two divisions--40,000 troops--of the former Korean Volunteer Army in China, who had trained under the Chinese communists, and had participated in the Chinese civil war (1945-49), returned to North Korea.

By June 1950, North Korean forces numbered between 150,000 and 200,000 troops, organized into ten infantry divisions, one tank division, and one air force division. Soviet equipment, including automatic weapons of various types, T-34 tanks, and Yak fighter planes, had also been pouring into North Korea in early 1950. These forces were to fight the ill-equipped South Korean army of less than 100,000 men--an army lacking in tanks, heavy artillery, and combat airplanes, plus a coast guard of 4,000 men and a police force of 45,000 men.

The events following the June 1950 invasion proved the superiority of North Korean military forces and the soundness of their overall invasion strategy. South Korea's army was simply overwhelmed; Seoul fell within three days. By early August, South Korean forces were confined in the southeastern corner of the peninsula to a territory 140 kilometers long and 90 kilometers wide. The rest of the territory was completely in the hands of the North Korean army.

The only unforeseen event complicating North Korea's strategy was the swift decision by the United States to commit forces in support of South Korea. On June 26, 1950, Truman ordered the use of United States planes and naval vessels against North Korean forces, and on June 30 United States ground troops were dispatched. The United States, fearing that inaction in Korea would be interpreted as appeasement of communist aggression elsewhere in the world, was determined that South Korea should not be overwhelmed and asked the UN Security Council to intervene. When Douglas MacArthur, the commanding general of the United Nations forces in Korea, launched his amphibious attack and landed at Inch'on on September 15, the course of the war changed abruptly. Within weeks much of North Korea was taken by United States and South Korean forces before Chinese "volunteers" intervened in October, enabling North Korea to eventually restore its authority over its domain. The war lasted until July 27, 1953, when a cease-fire agreement was signed at P'anmunjom. By then, the war had involved China and the Soviet Union, which had dispatched air force divisions to Manchuria in support of North Korea and had furnished the Chinese and North Koreans with arms, tanks, military supplies, fuel, foodstuffs, and medicine. Fifteen member-nations of the United Nations had contributed armed forces and medical units to South Korea.

The war left indelible marks on the Korean Peninsula and the world surrounding it. The entire peninsula was reduced to rubble; casualties on both sides were enormous. The chances for peaceful unification had been remote even before 1950, but the war dashed all such hopes. Sizable numbers of South Koreans who either had been sympathetic or indifferent to communism before the war became avowed anticommunists afterwards. The war also intensified hostilities between the communist and noncommunist camps in the accelerating East-West arms race. Moreover, a large number of Chinese volunteer troops remained in North Korea until October 1958, and China began to play an increasingly important role in Korean affairs. Because tension on the Korean Peninsula remained high, the United States continued to station troops in South Korea, over the strenuous objections of North Korean leaders. The war also spurred Japan's industrial recovery and the United States' decision to rearm Japan.