25 June 1950 · 57% of the peninsula north of the line
In August 1945, two American colonels split the Japanese surrender zones along the 38th parallel because it looked roughly halfway on a wall map. Five years later, before dawn on 25 June, around 90,000 North Korean troops crossed it behind Soviet-built T-34 tanks.
The South Korean army had no tanks, no heavy artillery and almost no air force. The line held for hours, not days.
38°N — the starting line, and near enough the finishing one. The dashed gold line stays on the map for the rest of the story.
5 July 1950 · 68% north of the line
Seoul was captured on 28 June. The first American ground unit into the fight, Task Force Smith, was flown in from Japan and brushed aside at Osan on 5 July. By the middle of the month the North Koreans held everything north of the Kum River and were closing on Taejon.
3 days from the border to the capital.
4 August – 15 September 1950 · 94% north of the line
By early August the UN held only a corner of the south-east behind the Naktong River: the Pusan Perimeter, a 230-kilometre arc around the port. For six weeks General Walker's Eighth Army shuttled its reserves along the arc to plug breaches at Taegu, Masan and Pohang.
The North Korean army had outrun its supply lines. Its strength had roughly halved on the way south.
15 September – 1 October 1950 · 56% north of the line
X Corps landed at Inchon, 240 kilometres behind the front, on 15 September. Seoul was back in UN hands by the 28th. Cut off from supply, the North Korean army came apart; the perimeter force broke out and drove north to meet the landing.
By 1 October the line was back where it had started — and South Korean troops crossed it heading north.
16 days to reverse ten weeks of retreat.
24 November 1950 · 18% north of the line
Pyongyang fell on 19 October. South Korean units reached the Chinese border at Chosan on 26 October; American troops reached it at Hyesan a month later. On 24 November MacArthur launched what he called the "home by Christmas" offensive.
Chinese troops had been crossing the Yalu in secret for five weeks, and had already mauled a US regiment at Unsan.
The line on the map is generous: the front was never continuous here. A 100-kilometre gap of mountains separated Eighth Army in the west from X Corps in the east.
25 November – 24 December 1950 · 56% north of the line
Around 300,000 Chinese troops struck on 25 November. Eighth Army fell back 200 kilometres in three weeks, the longest retreat in US Army history. In the east, X Corps fought its way out of the Chosin Reservoir in temperatures below −30 °C and was evacuated by sea from Hungnam, taking nearly 100,000 Korean refugees with it.
Pyongyang was abandoned on 5 December. By Christmas the line was back at the 38th parallel.
200 km lost in three weeks.
4 – 24 January 1951 · 67% north of the line
China's New Year offensive took Seoul on 4 January. General Ridgway, who had taken over Eighth Army after Walker died in a jeep crash, stopped the withdrawal on a line from Pyongtaek to Samchok, roughly the 37th parallel.
It was the deepest communist advance of the rest of the war — and a long way short of Pusan.
25 January – 21 April 1951 · 57% north of the line
Ridgway's counter-offensives were deliberate and firepower-heavy, advancing ten kilometres at a time. Seoul was retaken in mid-March, the last time it would change hands. By April Eighth Army stood on Line Kansas, just north of the parallel.
On 11 April Truman relieved MacArthur of command for publicly arguing for a wider war with China.
4th and final change of hands for Seoul.
22 April – 20 May 1951 · 61% north of the line
China committed around 700,000 troops in two waves aimed at Seoul. British troops on the Imjin and Australians and Canadians at Kapyong held the roads to the capital; the second wave, in the east, pushed nearly 50 kilometres before it ran out of food and ammunition.
~50 km — the deepest penetration of the spring offensive.
Mid-June 1951 · 56% north of the line
The UN counter-attack met an exhausted enemy and was back on Line Kansas by the end of May, with a bulge north to Line Wyoming at the Iron Triangle around Chorwon. On 10 July armistice talks opened at Kaesong, later moving to Panmunjom.
Both sides dug in. With talks under way, neither would risk a general offensive again.
56% — a share that would barely move for two years.
July 1951 – June 1953 · 56% north of the line
What followed was a war of trenches and outposts: Bloody Ridge and Heartbreak Ridge in the autumn of 1951, Old Baldy, White Horse and Triangle Hill in 1952, Pork Chop Hill in 1953. Each cost thousands of casualties and moved the line by a few kilometres, if at all.
The talks stalled for eighteen months on a single issue: whether captured Chinese and North Korean soldiers could refuse to go home.
25 months of fighting over the same ridgelines. The gold markers are the outpost battles.
27 July 1953 · 56% north of the line
After a final Chinese push at Kumsong in July, the armistice was signed at Panmunjom at 10 a.m. on 27 July 1953. The guns stopped twelve hours later. The Military Demarcation Line runs where the armies stood: the South ended the war with more ground in the east than it began with, the North with Kaesong and the Ongjin peninsula in the west.
No peace treaty has ever followed. The dashed gold line is the border of 1950; the solid line is the one that still divides Korea.
The share of the peninsula held by North Korean and Chinese forces swung from 57% to 94%, down to 18%, back up to 67%, and settled at 56%. Around three million people died, most of them Korean civilians.
The border they died over ended up almost exactly where it had been drawn on a wall map in 1945: one percentage point of the peninsula from where it started.
Frontline positions are redrawn by hand from US Army Center of Military History campaign maps and the West Point Atlas into lon/lat polylines. They are indicative, not surveyed. Each line represents the front on or around the date shown; the front was rarely continuous, and the November 1950 advance in particular had a wide gap between Eighth Army in the west and X Corps in the east. The shaded area is the land north of each line, closed along the coast and the Yalu, so it always follows the line exactly.
Share of the peninsula is the land area north of each line as a proportion of the whole peninsula (North and South Korea unioned, Natural Earth 1:10m). It is sensitive to exactly where a line meets the coast, so treat every figure as ±2 points. Frontline travel is the change in each line's mean latitude between beats, summed; it is a rough index of movement, not a distance marched.
Dates and operations follow the CMH campaign brochures, the Truman Library chronology, and the standard narrative histories: Halberstam's The Coldest Winter, Hastings's The Korean War and Fehrenbach's This Kind of War. Place names use the 1950s McCune-Reischauer forms Western readers know (Pusan, Inchon, Taegu) rather than Revised Romanisation (Busan, Incheon, Daegu).
A Vizmaya data story built from a research brief on the Korean War's moving frontline.