Korea · 1950–1953

How Did the Korean War Start?

By William Souef · Published August 19, 2026

A line drawn overnight

Korea's war begins with a decision made in haste. In August 1945 Japan was collapsing, Soviet troops were entering Korea from the north, and Washington wanted a demarcation for accepting Japanese surrender before the whole peninsula fell into the Soviet zone. Two colonels on night duty at the War Department — one of them Dean Rusk, a future Secretary of State — were told to propose a line. Working from a National Geographic map because nothing better was at hand, and with about half an hour, they picked the 38th parallel: it split the country roughly in half and left the capital, Seoul, on the American side. Moscow accepted it without argument.

Nothing about that line was meant to last. But by 1948 the Cold War had frozen it into two states, each claiming the whole peninsula: the Republic of Korea under Syngman Rhee in the south, the Democratic People's Republic under Kim Il-sung in the north. Both leaders wanted unification by force. Kim got permission first — Stalin approved his invasion plan in early 1950 after concluding, from American statements that placed Korea outside the defensive perimeter, that Washington would not fight.

Three years, four reversals

The invasion of 25 June 1950 nearly succeeded outright. Within two months, southern and American forces had been pushed into a small perimeter around Pusan at the peninsula's tip. Then MacArthur executed the amphibious landing at Inchon on 15 September, far behind the North Korean lines — an operation his own planners rated close to reckless, and which worked spectacularly. The North Korean army collapsed, and UN forces pushed north past the 38th parallel toward the Chinese border.

That advance produced the war's second transformation. China, having warned it would not accept hostile forces on the Yalu, entered in late October with hundreds of thousands of troops and drove the UN back below Seoul. By mid-1951 the front had stabilised roughly where it had started, and the remaining two years were a grinding positional war along fortified ridgelines while armistice talks stalled — largely over the repatriation of prisoners who did not want to go home.

Why it never ended

The armistice signed at Panmunjom on 27 July 1953 was a ceasefire between military commands, not a settlement between states. South Korea's president refused to sign at all. It created the Demilitarised Zone — a 250-kilometre strip that is, despite the name, among the most heavily armed borders in the world — and left every political question open. No peace treaty has followed in the seven decades since.

The war's consequences outgrew the peninsula. It militarised the Cold War globally: American defence spending tripled, NATO acquired a permanent integrated command, and the pattern of superpower confrontation through proxy wars was established for the next forty years. It also entrenched the division that produced two of the most divergent societies on Earth from one nation, and it is the reason a state of war technically persists on the 38th parallel today. In the United States it was long called the Forgotten War, sitting between the moral clarity of 1945 and the trauma of Vietnam; in Korea, nothing about it is forgotten.

Frequently asked questions

Why is Korea divided at the 38th parallel?

Because two American officers picked it in about half an hour in August 1945, using a National Geographic map, to split the peninsula into Soviet and American zones for accepting Japanese surrender. It had no historical or geographic basis; Seoul happening to fall on the southern side was the main criterion. The arrangement was explicitly temporary, and Koreans were not consulted.

Is the Korean War technically still going on?

Legally, yes. The fighting stopped with an armistice on 27 July 1953 — an agreement between military commands to cease hostilities — but no peace treaty has ever been signed, and South Korea declined to sign even the armistice. The Demilitarised Zone remains heavily fortified, and the two states remain formally at war.

Why did the war end almost exactly where it started?

Because both sides ran into the limits of what they could hold. The North nearly took the whole peninsula, the UN counter-offensive nearly reached China, and China's entry pushed the line back down again. By mid-1951 the front had settled near the 38th parallel, and two further years of positional fighting and stalled negotiations moved it very little — at a cost of hundreds of thousands more lives.