The Korean War was fought between North Korea and South Korea from 1950 to 1953. The war began on 25 June 1950 when North Korea invaded South Korea following clashes along the border and rebellions in South Korea. North Korea was supported by China and the Soviet Union while South Korea was supported by the United States and allied countries. The fighting ended with an armistice on 27 July 1953.

On 25 June 1950, the KPA crossed the 38th Parallel behind artillery fire. The KPA justified its assault with the claim that ROK troops attacked first and that the KPA were aiming to arrest and execute the "bandit traitor Syngman Rhee". Fighting began on the strategic Ongjin Peninsula in the west. In five days, the ROK, which had 95,000 troops on 25 June, was down to less than 22,000 troops. In early July, when U.S. forces arrived, what was left of the ROK were placed under U.S. operational command of the United Nations Command.

The Battle of Osan, the first significant U.S. 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 KPA at Osan but without weapons capable of destroying the KPA tanks. The KPA defeated the U.S. soldiers; the result was 180 American dead, wounded, or taken prisoner. The KPA progressed southwards, pushing back U.S. forces 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 its commander, Major General William F. Dean.

By September, UN forces were hemmed into a small corner of southeast Korea, near Pusan. This 230-kilometre (140-mile) perimeter enclosed about 10% of Korea, in a line partially defined by the Nakdong River. In the resulting Battle of Pusan Perimeter (August–September 1950), the UN forces withstood KPA attacks meant to capture the city at the Naktong Bulge, P'ohang-dong, and Taegu. To relieve the Pusan Perimeter, General MacArthur recommended an amphibious landing at Incheon, near Seoul and well over 160 km (100 mi) behind the KPA lines.

On 30 September, Zhou Enlai warned the U.S. that China was prepared to intervene in Korea if the U.S. crossed the 38th Parallel. By 1 October 1950, the UN Command repelled the KPA northwards past the 38th Parallel; the ROK advanced after them, into North Korea. In a meeting on 13 October, the Politburo of the Chinese Communist Party decided that China would intervene even in the absence of Soviet air support, basing its decision on a belief that superior morale could defeat an enemy that had superior equipment. Immediately on his return to Beijing on 18 October 1950, Zhou met with Mao Zedong, Peng Dehuai and Gao Gang, and the group ordered two hundred thousand PVA troops to enter North Korea, which they did on 19 October.

China justified its entry into the war as a response to what it described as "American aggression in the guise of the UN". Chinese decision-makers feared that the American-led invasion of North Korea was part of a U.S. strategy to invade China ultimately. They were also worried about rising counter-revolutionary activity at home. MacArthur's public statements that he wanted to extend the Korean War into China, and return the Kuomintang to power reinforced this fear.

After secretly crossing the Yalu River on 19 October, the PVA 13th Army Group launched the First Phase Offensive on 25 October, attacking the advancing UN forces near the Sino-Korean border. After inflicting heavy losses on the ROK II Corps at the Battle of Onjong, the first confrontation between Chinese and U.S. military occurred on 1 November 1950. On 25 November, on the Korean western front, the PVA 13th Army Group attacked and overran the ROK II Corps at the Battle of the Ch'ongch'on River, and then inflicted heavy losses on the U.S. 2nd Infantry Division on the UN forces' right flank. In the east, on 27 November, the PVA 9th Army Group initiated the Battle of Chosin Reservoir. Here, the UN forces fared comparatively better.

In early February, the ROK 11th Division ran an operation to destroy guerrillas and pro-DPRK sympathizers in Southern Korea. During the operation, the division and police conducted the Geochang massacre and Sancheong–Hamyang massacre. In mid-February, the PVA counterattacked with the Fourth Phase Offensive and achieved initial victory at Hoengseong. However, the offensive was soon blunted by U.S. IX Corps at Chipyong-ni in the center. The U.S. 23rd Regimental Combat Team and the French Battalion fought a short but desperate battle that bROKe the attack's momentum. The battle is sometimes known as the "Gettysburg of the Korean War": 5,600 South Korean, U.S., and French troops were surrounded on all sides by 25,000 PVA. UN forces had previously retreated in the face of large PVA/KPA forces instead of getting cut off, but this time, they stood and fought, and won.

In the last two weeks of February 1951, Operation Thunderbolt was followed by Operation Killer, carried out by the revitalized Eighth Army. It was a full-scale, battlefront-length attack staged for maximum exploitation of firepower to kill as many KPA and PVA troops as possible. Operation Killer concluded with U.S. I Corps re-occupying the territory south of the Han River, and IX Corps capturing Hoengseong.

The PVA counter-attacked in April 1951, with the Fifth Phase Offensive, with three field armies (approximately 700,000 men). The first thrust of the offensive fell upon I Corps, which fiercely resisted in the Battle of the Imjin River (22–25 April 1951) and the Battle of Kapyong (22–25 April 1951), blunting the impetus of the offensive, which was halted at the No-name Line north of Seoul. Casualty ratios were grievously disproportionate; Peng had expected a 1–1 or 2-1 ratio, but instead, Chinese combat casualties from 22 to 29 April totaled between 40,000 and 60,000 compared to only 4,000 for the UN —a casualty ratio between 10–1 and 15–1. By the time Peng had called off the attack in the western sector on 29 April, the three participating armies had lost a third of their front-line combat strength within a week.

For the remainder of the war, the UN and the PVA/KPA fought but exchanged little territory, as the stalemate held. Large-scale bombing of North Korea continued, and protracted armistice negotiations began on 10 July 1951 at Kaesong, an ancient capital of Korea located in PVA/KPA held territory. On the Chinese side, Zhou Enlai directed peace talks, and Li Kenong and Qiao Guanghua headed the negotiation team. Combat continued while the belligerents negotiated; the goal of the UN forces was to recapture all of South Korea and to avoid losing territory. The PVA and the KPA attempted similar operations and later effected military and psychological operations to test the UN Command's resolve to continue the war.

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The on-again, off-again armistice negotiations continued for two years, first at Kaesong, on the border between North and South Korea, and then at the neighboring village of Panmunjom. A major, problematic negotiation point was prisoner of war (POW) repatriation. The PVA, KPA and UN Command could not agree on a system of repatriation because many PVA and KPA soldiers refused to be repatriated back to the north, which was unacceptable to the Chinese and North Koreans. A Neutral Nations Repatriation Commission, under the chairman Indian General K. S. Thimayya, was subsequently set up to handle the matter.

Under the Armistice Agreement, the belligerents established the Korean Demilitarized Zone (DMZ), along the front line, which vaguely follows the 38th Parallel. In the eastern part, the DMZ runs north of the 38th Parallel; to the west, it travels south of it. Kaesong, site of the initial armistice negotiations, was originally in pre-war South Korea but now is part of North Korea. The DMZ has since been patrolled by the KPA and the ROK and the U.S. still operating as the UN Command.