The Korean War “created a blood debt that is crucial to understanding North Korean behaviour ever since”

Which countries fought in the Korean War?

Although some 16 nations fought under the United Nations flag, the Korean War was primarily between South Korea and the US on the one hand, and North Korea and China on the other.

When did the conflict start and end?

The conventional American story – that is, the official story – is that the war began on 25 June 1950 and ended on 27 July 1953. But this war has deep origins in 20^th-century history – more on this below…

What are the origins of the Korean War?

The foundations of the war date back to the 1930s. The founder of North Korea, Kim Il-sung, began a guerrilla struggle against Japanese forces who had invaded the three north-east provinces of China in September 1931 and proclaimed the puppet state of Manchukuo on 1 March 1932. The North Koreans trace the inception of their army to battles that began the next month, in April 1932.

Kim Il-sung and his comrades fought for the next decade in completely inhospitable terrain, where temperatures fell to -40 ̊C in the winter. An assortment of guerrilla groups were part of this struggle, with some sources alleging that Kim Il-sung’s group was commanded by Chinese Communists. In fact, most of the guerrillas were Korean, and Koreans were even the majority in the so-called Chinese Communist Party. Korean commanders did what they wanted and were not part of a Chinese hierarchy. These guerrilla groups bedevilled Japanese forces, bogging them down in an unwinnable war.

Things came to a head in 1939, with pitched battles involving tens of thousands of Japanese troops. By 1941 the guerrillas had been seriously depleted, and they withdrew to training camps near the Sino-Russian border, in the vicinity of Khabarovsk, to await the inevitable outcome of the United States joining the fight against Japan after the attack on Pearl Harbor.

• Caught off guard: Why didn’t America see Pearl Harbor coming?

The significance of this history is twofold: firstly, it constitutes the founding myth of North Korea, after about 200 surviving guerrillas returned to Pyongyang in 1945 and became the elite group that ruled the nation. This group is still in power today, but it is considerably larger after 75 years of ruling exclusively.

The other crucial fact of the 1930s is that the Japanese employed Korean officers to go after the guerrillas, and these same officers populated the upper ranks of the South Korean army in the 1940s. Take Kim Sok-won, for instance: a colonel in the Japanese army who had been given the task of chasing down Kim Il-sung. Kim Sok-won, by then a general, was the commander of the 38^th parallel throughout the summer and autumn of 1949. It was this conflict between Koreans who chose opposite sides in the 1930s that made civil war highly likely.

Another factor that led to the outbreak of the Korean War was that, after the collapse of Japanese rule in August 1945, ordinary Koreans began setting up political committees to run local affairs on a spontaneous basis. Soviet troops who were occupying the north supported these committees, and these political groups eventually became the basis of the North Korean regime, right down to the present. American troops arrived on 8 September 1945 and set up a three-year military government. In some parts of South Korea, the US worked with the committees, but in other parts they sought to suppress them, throwing the leaders in jail. This led to an open revolt in the fall of 1946; an inquiry after this revolt revealed that the US was using Korean members of the hated Japanese colonial police throughout the territory.

• Listen| Historian Grace Huxford describes the key events of the Korean War and explains how it played out in Britain

Most of the committees were underground by 1948, but they continued to govern on Cheju Island, where the committees were largely left to their own devices. On 3 April 1948, an uprising on the island against a plan to divide the two Koreas led to a complete bloodbath over the next two years, with 10 per cent of the islanders – about 30,000 people – being killed by national police, military and members of right-wing youth groups who had been expelled from North Korea. The suppression forces were under the command of Korean officers who had served in the Japanese military.

The history of this conflict was buried for decades under the dictatorships in South Korea, but in recent years it has become a kind of touchstone, prefiguring the civil war to come. It was inconceivable that the North Korean leaders, and their supporters in the south, would allow this slaughter to go unpunished.

• Why did North Korea go nuclear?

What happened next?

The immediate crucible of the coming war was the fighting along the border in the summer and autumn of 1949. In August 1945 American planners had chosen the 38^th parallel as an appropriate line to mark the respective American and Soviet spheres. They consulted no one – not their allies, not the Soviets, and not a single Korean. The United States had operational control of the fledgling South Korean army until 30 June 1949, when the last American combat troops were withdrawn – leaving behind a 500-man military advisory group.

Fighting along the parallel had begun a month earlier, in May 1949. According to the US commander, it was sparked by the south; he said the south started more than half of the border fighting in 1949. A war nearly broke out in early August 1949, but both the US and the Soviet ambassadors intervened to restrain hotheads. The last Southern attack across the border came in December 1949, and then the parallel quieted for six months.

The north was not ready to fight in 1949, because tens of thousands of its crack troops were still fighting on the side of the Communists in the Chinese Civil War. But in the following months, they filtered back into North Korea and became the spearhead of Kim Il-sung’s invasion force in June 1950. The six-month gap in fighting made the invasion look like unprovoked aggression: it was, in the words of the US ambassador, “fortunately clear-cut”.

How did the Korean War start?

The start of the Korean War as conventionally understood is easily depicted. The North Koreans flowed down the peninsula in July and August 1950 – in spite of the American President Harry Truman sending ever larger numbers of troops. Finally, the US Marine First Brigade was able to stabilise the front in the south-east, which became known as the Pusan Perimeter. That made possible a dramatic landing at the port of Inchon, under the command of General Douglas MacArthur.

The US pounded North Korea from the air, leaving a blood debt that remains crucial to understanding North Korean behavior ever since

Within two weeks American leaders decided to invade North Korea on a ‘roll-back’ mission. US troops went all the way to the Yalu River, on the border with China, only to be thrown back in a massive campaign by Chinese and North Korean troops. By 1 January 1951, Seoul was again occupied by Chinese and North Korean troops. By May, however, Seoul had been recaptured, and the fighting had stabilised roughly along what is now the demilitarized zone (DMZ). There followed two years of trench warfare and truce negotiations, until the armistice was signed on 27 July 1953.

Throughout the war the US pounded North Korea from the air, leaving hardly a modern building standing, killing untold numbers of civilians, dropping so much napalm that even Winston Churchill complained about it, and leaving a blood debt that remains crucial to understanding North Korean behaviour ever since.

How many deaths and casualties were there?

33,686 American troops were killed in action, and the UK and Australia also suffered significant casualties: over 1,000 and 339 soldiers died, respectively. But Koreans and Chinese lost their lives in far greater numbers. Just under a million Chinese were killed, about the same number of South Koreans, and perhaps two million North Koreans.

• Forging alliances: Britain and the US in the Korean War

How did the Korean War end?

The war ended about where it began: in a stalemate with no real winner. China enjoyed an enhanced status, having fought the US to a standstill, and the two shattered Koreas were left to somehow rebuild their nation.

It took two years to negotiate the armistice, and after decades of work toward a peace treaty starting at Geneva in 1954, none has come to fruition. But the armistice has done its work in holding the peace, give or take some violent incidents from time to time.

Why is it called ‘the forgotten war’?

In the US the Korean War quickly became a so-called ‘forgotten war’, in part because it was such a contrast with the halcyon years under Eisenhower in the 1950s – and of course, because the US did not win for the first time in its history, going back to the War of 1812 stalemate. Some 30 years later a sombre Korean War Memorial was built not far from the Lincoln Memorial, with many individual depictions of how terrible this war was carved onto the faces of soldiers.

Within two short years the US had committed itself to the Vietnamese government in Saigon, and against the revolutionary forces of Ho Chi Minh. Soon the US was again fighting anti-colonial armies, while relying on Vietnamese officers who had served the French. Both of these wars were fundamentally anti-colonial in nature, but American leaders simply never could understand that relying on colonial quislings doomed both efforts from the start.

• Your guide to the Vietnam War

What was the significance of the Korean War?

It is hard to say what the significance of this war was for Koreans. Nothing was really solved, and the national division acquired a tragic permanence. Perhaps the foreign alliances that came with the war were critical – South Korea with the US, and North Korea with China.

But this war had tremendous significance for Americans: defence spending quadrupled as the US took on a mission to contain communism anywhere in the world; a national security state at home managed hundreds of permanent military bases abroad; a large standing army now existed in peacetime for the first time in American history; and a sprawling, hugely funded CIA was a font of power under Allen Dulles, the director (whose brother, John Foster Dulles, was Secretary of State under Eisenhower).

The Korean War also gave a huge boost to both the American and the Japanese economies, with war procurements leading some to call the effort “Japan’s Marshall Plan”. It all probably would have happened anyway, had the war occurred elsewhere. But it didn’t: it happened in Korea, giving the war tremendous significance in the American psyche.

Professor Bruce Cumings teaches in the history department at the University of Chicago and is the author of The Korean War: A History (Random House, 2010)