On June 25, 1950, after some initial fighting in the Ongjin area, North Korea launched a full-scale invasion across the 38th parallel. North Korean invasion forces, which consisted of 90,000 troops and supported by armored and artillery units, crossed into South Korea from east to west of the line. South Korean border defenses south of the line were easily overcome. South Korean forces, lacking heavy artillery and powerful anti-tank weapons, surrendered or defected en masse, or fled south. On June 28, 1950, Seoul fell, with President Rhee and his government having vacated the capital in advance of the North Korean offensive. To forestall the North Koreans, the South Korean military destroyed the main bridge south of Seoul across the Han River, causing the deaths of hundreds of civilians who were crossing the bridge at the time. Thousands of South Korean troops also were unable to leave the city and were later captured by the North Koreans. By the third day of the invasion, South Korea was verging on collapse.
On June 25, 1950, the United Nations Security Council (UNSC) passed Resolution 82 which called for an end to hostilities, and demanded that North Korean forces withdraw from South Korea. The resolution passed because at that time, the Soviet Union, which was a permanent UNSC member with veto power, had boycotted the UNSC meetings in protest of the UN’s continued non-recognition of China. The Soviet government also challenged the UN’s legitimacy to decide on the Korean conflict, stating that the war was an internal security issue, and that the 38th parallel was a military demarcation and not an international border.
Then on June 27, 1950, the UNSC passed Resolution 83, which called on UN member states to provide military assistance to South Korea to counter the North Korean invasion. Like South Korea, the United States was caught off-guard by the invasion, but quickly moved into action, and used its strong diplomatic influence to mobilize international condemnation of North Korea. Up until now, President Truman viewed the Cold War as relating only to Europe, and the U.S. containment policy as directed against the Soviet Union and its Eastern Bloc allies. But with the outbreak of the Korean War, for the United States, the Cold War had come upon Asia.
President Truman particularly likened the North Korean invasion to Germany’s aggression in World War II, and announced that his government would not repeat the pre-war Allied appeasement policy, and that the United States would meet the North Korean “challenge” with force. And in view of this expanded Cold War policy, on June 27, 1950 (two days after the start of the Korean War), President Truman ordered the U.S. 7th Fleet to proceed to the Taiwan Strait, to prevent hostilities between the Republic of China (Taiwan) and the People’s Republic of China. Both Chinese states pursued a policy to destroy its rival, but the arrival of the American naval fleet deterred the People’s Republic of China from launching its long-planned invasion of Taiwan.
At the start of the war, the U.S. military was undergoing a drastic reduction in combat strength because of major cutbacks in military appropriations following World War II. Furthermore, because of the perceived greater security threat in Europe, the United States concentrated its forces there in line with its “Europe First” policy, leading to the U.S. military scrambling to assemble enough American military units for Korea. During the early stages of the war, the United States experienced some difficulty dispatching sufficient forces to the fighting, as U.S. units in Japan were insufficiently trained for combat and seriously under-strength. As a result, only small advance units initially were sent to the fighting in Korea.
Subsequently during the war, a combined total of some 370,000 foreign troops from 16 UN countries fought on the side of South Korea. Of this number, nearly 90% was provided by the United States (326,000 troops), while Britain (14,000), Canada (8,000), and Turkey (5,000) also sent sizable contingents.
On the day of the invasion, the U.S. government evacuated American civilians from South Korea. On June 27, 1950, with the passage of UNSC Resolution 83 authorizing the use of force against the North Korean invasion, the U.S. military based in Japan sent warplanes into bombing raids in North Korea. U.S. planes attacked airfields and destroyed several North Korean planes on the ground. U.S. ships also were rushed to Korean waters, where they shelled North Korean positions along the coast, somewhat slowing down the North Korean advance in these coastal areas.
On July 7, 1950, the UNSC passed Resolution 85, which merged all UN member units into one unified force (called the United Nations Command, or UNC) under one commander. That same day, President Truman named General Douglas MacArthur (head of the Far East Command based in Tokyo, Japan) as commander-in-chief of the UNC. The Eighth U.S. Army, headquartered in Japan, would serve as the main American force in the Korean War. Its commander, General Walton Walker, was named as commander of the UNC ground forces. The South Korean government, whose army had been reduced to 22,000 troops from 90,000 since the start of the war, allowed its remaining forces to be placed under the UNC.
On July 1, 1950, the first U.S. force, a 400-man battalion called Task Force Smith (named after its commander), arrived in Korea. Four days later, July 5, Task Force Smith encountered an armored North Korean column consisting of 5,000 troops that was advancing toward Osan. In the ensuing battle, Task Force Smith caused some material damage to the enemy, destroying a number of North Korean tanks, but itself was decimated and forced into a chaotic retreat. But Task Force Smith achieved one objective: it delayed the North Korean advance to allow time for more UN forces to arrive in the southern edge of the peninsula, where they could establish a stronger defensive line.
Meanwhile, other American units that had arrived in Korea also used delaying tactics in clashes at Pyeongtaek, Cheonan, Chochiwon, and Taejon, but were forced to retreat by numerically superior North Korean forces that advanced using both frontal attacks and flanking tactics. At Taejon particularly, the U.S. 24th Infantry Division was nearly destroyed and its commander, Major General William Dean, was captured by the North Koreans. By this time, South Korean and UN forces had been pushed to nearly the southern edge of the Korean Peninsula and faced the danger of being annihilated or driven to the sea.
However, in early August 1950, UN forces succeeded in establishing the Pusan Perimeter (Figure 16), a 140-mile long defensive line that partially followed the length of the Naktong River. The Pusan Perimeter was so-named for Pusan, South Korea’s major southern port, where U.S. and other UN forces, together with their war materials, were arriving in large numbers daily.
In August 1950, North Korean forces attacked many points along the Pusan Perimeter, and heavy fighting took place in Taegu, Masan, P’ohang-dong, and across the Naktong River. Because UN forces yet were numerically inadequate to defend the whole line, General Walker used a “mobile defense” strategy, where his forces were moved constantly to areas of enemy attack. North Korean forces broke through in many places, including a flanking maneuver that threatened to drive straight to Pusan. But UN forces succeeded in establishing new defensive positions and then counterattacked, driving back the North Koreans.
By early September 1950, North Korean forces were experiencing supply problems, as UN (mainly American) planes, which controlled the skies, were taking a heavy toll on North Korean logistical lines, attacking North Korean rail and road networks, weapons depots, oil refineries, and military facilities. As well, UN forces now had 180,000 troops. By contrast, the North Korean invasion force, which had experienced heavy casualties, stood at some 100,000 troops with the arrival of more reinforcements. UN forces also now had 600 tanks while North Korean armor, which had spearheaded the invasion, had been reduced to fewer than 100 tanks from the original 270 at the start of the war.
Nevertheless, in early September 1950, North Korean forces, comprising 13 divisions, launched an all-out coordinated five-prong offensive in a desperate attempt to finally break through the Pusan Perimeter. Intense fighting took place in Haman, Kyongju, Yongsan Tabu-Dong, and Ka-san. The North Koreans, using the element of surprise and exceptionally fierce attacks, pushed back the UN forces in many places. But by mid-September 1950, UN forces had succeeded in re-establishing a new defensive line, although the situation remained critical.
As early as July 1950, General MacArthur had conceived of a plan to launch a UN amphibious assault at Inchon harbor, located 27 kilometers southwest of Seoul on the central west coast. The success of such an operation would have the strategic effect of destabilizing the North Korean supply lines to the south, and threaten the North Korean forces fighting in the Pusan Perimeter. The U.S. Joint Chiefs of Staff (JCS) initially were skeptical about the operation because of the risks involved, but soon gave its approval when General MacArthur expressed unwavering optimism in the feasibility of his plan. U.S. forces then prepared to launch an amphibious landing on Inchon.
On September 15, 1950, preceded by days of heavy air attacks and naval artillery bombardment, some 75,000 U.S. and South Korean troops (of the newly reconstituted U.S. X Corps) in 260 naval vessels were amphibiously landed north and south of Inchon, taking the city where they met only light resistance from the small North Korean garrison.
The unexpected UN landing at Inchon dealt a psychological blow to North Korean forces at the Pusan Perimeter. Already weakened by shortages of food and ammunitions, and rising casualties, by the third week of September 1950, North Korean resistance collapsed, with whole military units breaking down, and tens of thousands of troops fleeing north or to the mountains, or surrendering en masse. For the North Korean Army, its defeat at the Pusan Perimeter was catastrophic: some 65,000 (over 60%) of its 98,000 troops were lost; it had lost nearly all its tanks and artillery pieces; and most crucially, it ceased to be a force capable of stopping the UN forces which now began to steamroll northward.
By September 23, 1950, UN forces, comprising largely of the Eighth U.S. Army, had broken out of the Pusan Perimeter, and advanced north some 100 miles, on September 27 linking up with X Corps units from the Inchon landings at Osan. However, the UN forces’ aim of linking their units rather than actively pursuing the enemy allowed some 30,000 retreating North Korean soldiers from the Pusan Perimeter to escape and eventually cross the 38th parallel into North Korea, where they soon were reorganized into new fighting units. Other North Korean units that took to the mountains in the south also formed small militias that engaged in guerilla warfare.