On June 25, 1950, North Korean forces invaded South Korea, turning a tense Cold War division into open war. The United States quickly treated the attack as a major test of the post–World War II security system built around the United Nations. This invasion set the stage for U.S. military intervention under UN authority.
That same day, the UN Security Council adopted Resolution 82, condemning the invasion and calling for an immediate halt to hostilities and withdrawal to the 38th parallel. This early UN action helped frame the conflict as aggression against a UN member state rather than a declared U.S. war. It also created a legal and political foundation for later U.S.-led collective action.
On June 27, 1950, President Harry S. Truman ordered U.S. air and naval forces to support South Korea. He presented the move as enforcing UN decisions and resisting armed aggression without asking Congress for a formal declaration of war. This decision committed the United States to a major conflict in East Asia under the label of a UN “police action.”
Also on June 27, the UN Security Council adopted Resolution 83, recommending that UN members furnish assistance to South Korea to repel the attack. This step strengthened the international basis for U.S. action and encouraged other countries to contribute forces. It also tied U.S. military decisions in Korea to a broader UN command structure.
On July 5, 1950, Task Force Smith fought North Korean forces near Osan in the first ground combat involving U.S. troops in the war. The engagement showed how unprepared early U.S. units were for modern armor and large-scale attacks. The setback helped drive urgent decisions to send more U.S. forces and equipment to Korea.
On July 7, 1950, the UN Security Council adopted Resolution 84, recommending that military contributions be placed under a unified command led by the United States. It also authorized the commander to use the UN flag. This formalized U.S. leadership in the war while preserving the political identity of a UN-backed operation.
By August 1950, U.S. and South Korean forces had fallen back to a defensive line in the southeast, known as the Pusan (Busan) Perimeter. Holding this perimeter prevented the collapse of South Korea and bought time for reinforcements and planning. The defense became a turning point that made a later counteroffensive possible.
On September 15, 1950, U.S.-led UN forces carried out an amphibious landing at Inchon (Operation Chromite). The surprise landing cut North Korean supply lines and helped recapture Seoul, changing the momentum of the war. It also encouraged U.S. leaders to consider moving beyond simply defending South Korea.
On October 15, 1950, President Truman met General Douglas MacArthur at Wake Island to discuss the war’s direction as UN forces advanced north. The meeting highlighted growing tension between civilian leaders seeking a limited war and military leaders pushing for broader victory. These disagreements later became central to a major U.S. political crisis.
In October 1950, Chinese forces (the “People’s Volunteer Army”) entered Korea, and by late October they began major engagements against UN units. Chinese intervention overturned UN advances in North Korea and led to a severe winter retreat. For U.S. policymakers, the intervention raised the risk that a regional war could widen into a larger conflict.
From November 27 to December 13, 1950, UN forces—especially U.S. Marines—fought a brutal breakout at the Chosin Reservoir in freezing conditions. The fighting led to a major withdrawal and evacuation by sea from northeast Korea. The campaign showed how Chinese intervention had changed the war from a rapid advance to a struggle to avoid defeat.
On April 11, 1951, President Truman removed General MacArthur from his commands after repeated public disputes over war policy, including proposals to expand the conflict. Truman argued that military commanders must follow elected civilian leadership. The decision reaffirmed U.S. constitutional control of the military and signaled continued commitment to a limited war in Korea.
On July 10, 1951, armistice talks began at Kaesong as front lines stabilized near the prewar dividing area. Negotiators faced major disputes, especially over prisoners of war and whether repatriation should be voluntary. The talks marked a shift from fast-moving offensives to a long war of positions combined with diplomacy.
On July 27, 1953, the Korean Armistice Agreement was signed at Panmunjom by representatives of the United Nations Command, North Korea, and China. The agreement created a ceasefire and established the Demilitarized Zone (DMZ), but it was not a peace treaty. The war’s main fighting ended, yet the political conflict remained unresolved.
Beginning in early August 1953, the two sides carried out Operation Big Switch, returning large numbers of prisoners of war after the armistice. Prisoner exchanges were politically important because the sides disagreed over forced versus voluntary return. The exchanges helped turn the ceasefire into a more stable postwar situation, even without a final peace settlement.
Korean War: U.S. Military and Political Actions in Korea (1950–1953)