1. Korea was split in half after World War II.

Japan ruled over Korea from 1905 until the end of World War II, after which the Soviet Union occupied the northern half of the peninsula and the United States occupied the south. Originally, they intended to keep Korea together as one country. But when the United Nations called for elections in 1947, the Soviet Union refused to comply, instead installing a communist regime led by Kim Il-Sung. In the South, meanwhile, strongman Syngman Rhee became president. Both Kim and Rhee wanted to unify Korea under their rule and initiated border skirmishes that left thousands dead.

2. The U.S. Congress never declared war, thereby establishing a precedent.

On June 25, 1950, North Korea launched a full-scale invasion of the South after receiving the go-ahead from Soviet dictator Joseph Stalin. Until that point, the United States appeared disinclined to intervene. The year before it had removed its last remaining troops from Korea, and that January U.S. Secretary of State Dean Acheson had given a speech in which he excluded Korea from America’s defense perimeter. Nonetheless, the United States got involved in the Korean War almost immediately. On June 27, as North Korean troops reached the outskirts of Seoul, the South Korean capital, U.S. President Harry S. Truman ordered naval and air forces into action. Then, three days later, he approved the use of ground troops. Though the Constitution gives Congress the power to declare war, it did not do so in this case. “If a burglar breaks into your house, you can shoot him without going down to the police station and getting permission,” Senator Tom Connally told Truman when asked if Congressional approval was necessary. As a result, Congress has not formally declared war since World War II.

3. The United Nations played a major role.

On the first day of the war, the United Nations Security Council demanded that North Korea stop fighting and withdraw to the border along the 38th parallel. When this warning was ignored, it passed a second resolution asking its member states to assist South Korea in repelling the attack. It then established a unified command under U.S. General Douglas MacArthur, who had gained fame fighting in the Pacific during World War II. All of these measures would have been vetoed by the Soviet Union had it not been boycotting Security Council meetings at that time. In the end, although the United States and South Korea provided most of the manpower and military equipment, 15 countries fought with them, including the United Kingdom, France, Australia, Canada, Turkey and Ethiopia.

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4. Long retreats marked the early stages of the war.

The invading North Korean troops were able to capture Seoul within three days—the first of four times that city would change hands—and quickly pushed their opponents back to the so-called Pusan Perimeter in the extreme southeastern portion of the peninsula. The tide turned that September, however, in part due to a surprise amphibious landing that General MacArthur executed behind enemy lines at Inchon, the port for Seoul. U.N. troops then pushed deep into North Korea, precipitating rumors that they would be home by Christmas. When China entered the war in October on the side of North Korea, MacArthur initially discounted its importance. If they tried to advance, then “there would be the greatest slaughter,” MacArthur told Truman. He was quickly proven wrong, as a November attack by the Chinese sent the overextended American troops into the longest retreat in U.S. military history, nicknamed the “Big Bug-Out.” “They turned our Army into a leaderless horde, running headlong for Pusan,” one soldier later wrote. The final substantial retreat of the war came in spring 1951, when U.N. troops, having repelled a major communist offensive, advanced a short distance into North Korea.

5. MacArthur was fired for insubordination.

Following the “Big Bug-Out,” the Truman administration abandoned its goal of unifying Korea and expressed its willingness to negotiate with the Communists. But MacArthur continued advocating for an escalation. In December 1950 he stated that Washington’s refusal to allow him to attack bases in China was “an enormous handicap, without precedent in military history.” He also wanted to blockade China and bring Chinese Nationalists from Taiwan into the conflict, among other “all-out measures.” Then, on April 5, 1951, a letter from MacArthur was read on the floor of the U.S. House of Representatives, in which he declared that there was no substitute for victory. Truman relieved him of command a few days later. “We are trying to prevent a world war—not to start one,” Truman said at the time. MacArthur returned to the United States a hero in the eyes of many, but never gained much traction during his flirtation with a 1952 presidential bid.

6. Truce talks went on for most of the war.

Official ceasefire negotiations began in July 1951, by which time the wild swings of the early war had been replaced by limited attacks on strategic positions. Within months the two sides had agreed to divide the country along the existing battle line and not the 38th parallel. This would give South Korea slightly more territory than it had before the war. Fighting temporarily died down at the front as a final deal appeared imminent. But it was held up by a dispute over the repatriation of prisoners of war. The Communists wanted all POWs to be forcibly sent home, whereas the United States wanted them to be able to choose. Finally, after Stalin’s death in March 1953, the Communists conceded on this issue, which led to 14,704 Chinese, 7,900 North Koreans, 335 South Koreans, 23 Americans and one Briton refusing repatriation following the signing of the armistice.

7. The U.S. military integrated during the war.

In July 1948 President Truman desegregated the military with an executive order that mandated “equality of treatment and opportunity” for all soldiers. Even so, separate black units remained the norm at the start of the Korean War. Piecemeal integration came when, as casualties mounted, field commanders of white units began accepting black replacements. More institutional change then occurred once General Matthew B. Ridgway, who called racial segregation “both un-American and un-Christian,” took over for MacArthur. By May 1952 he had integrated the entire Far East Command, and by September 1954 the rest of the armed forces had followed suit.

8. No permanent peace treaty has ever been signed.

The July 1953 armistice may have ended the war, but it has not led to a peace treaty between North and South Korea. The two sides are still separated by a heavily fortified 2.5-mile-wide demilitarized zone, and tensions remain high, particularly over the North’s fledgling nuclear weapons program. North Korea has also occasionally resorted to assassination attempts and border incursions, including a 2010 artillery attack against a South Korean island that left four dead. Though North Korea has pronounced the armistice nullified on several occasions, most recently this March, the United Nations holds that such action cannot be taken unilaterally.