The Korean War: The Proxy War That Nearly Sparked World War III

On the morning of June 25th, 1950, as the sun rose over the Taebaek Mountains, 75,000 troops from the Korean People’s Army, spearheaded by 120 Soviet T-34 tanks, descended from North Korea into South Korea. They moved quickly and efficiently, capturing many of the country’s critical military positions before its government could react. South Korea had completely failed to anticipate the attack, and its inadequate army was easily overwhelmed.

It did not even possess the anti-tank weaponry needed to repel a threat of that magnitude. Within hours, every South Korean soldier along the border had been killed, captured, or pushed further south.

The invasion carried on into the next day, and the North Koreans continued to devastate the ill-prepared, ill-equipped South Koreans. On June 27th, the Korean People’s Army arrived outside Seoul. South Korea’s leader, Syngman Rhee, was forced to evacuate after heavy resistance, and the following day the capital fell.

In just five disastrous days, South Korea’s army had lost more than 75% of its armed forces, 60% of its military equipment, and numerous invaluable positions along its northern border. By the sixth day, it was in a near-total retreat. Across the Pacific, the United States—which had learned of the invasion roughly 12 hours after it began—was already readying its forces to intervene, fearing that if South Korea fell, Japan and the rest of the Pacific could follow it under communist control.

Over the next three years, the United States and the United Nations would collide with North Korean and Chinese forces in the first of several Cold War proxy conflicts fought between the world’s two largest superpowers. Had things played out differently, this analysis argues, the war could have escalated into something far worse. In North Korea it is remembered as the Fatherland Liberation War, and in South Korea as the 6-2-5 Upheaval—but had certain military leaders had their way, it might instead be remembered as the conflict that sparked the Third World War, and the world’s first nuclear war.

Forging a New Korea

To understand why the invasion happened, it is necessary to step back and examine why the Korean Peninsula was split, and how tensions between north and south originated. In 1910, the once-independent Korean Empire was formally annexed by imperial Japan, and its emperor, Gojong, was deposed. Over the next 35 years, Japan ruled Korea and harvested its natural resources to build and maintain an ever-growing army—the same army it would use to enter the Second World War on the side of the Axis Powers.

In August 1945, roughly three weeks before Japan’s unconditional surrender marked the formal end of the war, Japanese-occupied Korea was liberated by the Soviet Union from the north and the United States from the south. Though technically allies during the later years of World War II, both nations had an interest in occupying the peninsula. Both understood that a well-established foothold there was key to controlling the Pacific islands, including—most importantly—Japan. To resolve the dispute, a line was drawn along the 38th parallel: the Soviets would occupy everything north of it, the Americans everything to the south.

Two Governments, One Peninsula

Both the Americans and the Soviets claimed they intended to someday reunify Korea and let its people govern themselves. In practice, both powers moved quickly to install local governments that mirrored their own. In the north, the Soviet-backed Democratic People’s Republic of Korea, or DPRK, was founded under Kim Il-Sung. In the south, the US-backed Republic of Korea, or ROK, was founded under Syngman Rhee.

Once leadership was in place, both Kim Il-Sung and Syngman Rhee began claiming ownership over the other’s land, each asserting that the rival government’s claims—and the rival government itself—were illegitimate. Given the circumstances by which both states had been founded, the two claims were, technically, equally legitimate, and it soon became clear there would be no simple solution to the problem the Soviets and Americans had created for the Korean people.

Matters worsened as the relationship between Washington and Moscow deteriorated. Without a world war to bind them together, the two countries’ ideological, economic, and political differences rapidly drove a wedge between their leaders. Neither the US nor the Soviet Union, by their own admission, wanted any action that might trigger a full-scale war between them. Both were now nuclear powers and knew such a war would be catastrophic.

Kim Il-Sung and Syngman Rhee, by contrast, were each eager to reunify the peninsula—even at the risk of dragging their allies into the fighting.

Securing Stalin’s Approval

By 1949, skirmishes along the 38th parallel had collapsed negotiations between North and South Korea, pushing Kim Il-Sung to seek Joseph Stalin’s approval for an invasion. Drawing on the intelligence he had received, Kim told Stalin that domestic uprisings in the south had weakened Rhee’s hold over the country and reduced his military to under 100,000—far fewer than Kim’s own fighting force, which numbered close to 200,000 and was still growing.

Stalin would have welcomed a unified Korea flying the banner of communism, but he was initially hesitant to throw the Soviet Union’s full weight behind the plan. The Chinese civil war was still ongoing, and US forces stationed in the south meant the DPRK would be attacking not only South Korea’s army but America’s as well.

Months later, the calculus changed. The Chinese civil war concluded with Mao Zedong’s rise to power, and the United States withdrew the majority of its troops from the peninsula. Stalin reconsidered. He agreed to support the invasion with military aid—but only if China committed to sending ground troops should the war turn against the North.

He also made it unmistakably clear that the Soviet Union would not engage US troops in open combat, to avoid a full-scale nuclear war. Kim Il-Sung accepted these terms, China agreed, and the plan was set.

Heavy Losses and a Race Against Reinforcements

After suffering massive casualties at the hands of the Korean People’s Army, or KPA, the Republic of Korea Army, or ROKA, fled south in a near-total retreat during the war’s opening days. Determined to keep his momentum, Kim Il-Sung ordered the KPA to keep advancing, keep overrunning ROKA positions, and keep pushing Rhee’s forces toward the sea. His goal was to force an unconditional surrender before the United States and the United Nations could organize meaningful reinforcements—and his plan nearly worked.

In less than a week, the KPA had wiped out more than 75% of ROKA’s forces, and the few US troops who had arrived to help were underequipped and unable to stall the advance. After a month, Rhee had lost over 90% of the territory he once controlled. His forces were backed into a corner all the way to the port of Pusan, rapidly running out of ground to defend. The victories led Kim Il-Sung to boast that he could achieve a decisive win by the end of August, roughly two months after the invasion began.

The rest of the world, however, was not standing idly by. As word spread, both the US and the UN were shocked by how quickly the KPA had seized control of the battlefield—yet many believed Kim’s army was not as formidable as its victories suggested. They judged his success the product of favorable variables: North Korea had genuinely been better prepared, with vastly superior communication and supply lines and months of readying its troops, and it had enjoyed the all-important element of surprise. Kim Il-Sung had capitalized on a moment of vulnerability, and many leaders believed a strong resistance would quickly undo his gains.

Truman, the Domino Fear, and MacArthur’s Command

President Harry S. Truman grew concerned that the attack had been encouraged—or even orchestrated—by China or the Soviet Union. That was not the case; Kim Il-Sung was the architect of the invasion. But to Truman it scarcely mattered.

He saw a direct parallel between North Korea’s assault on the South and Germany’s 1939 invasion of Poland, and, determined not to repeat the mistakes of the past, he refused to consider appeasement. Communism was spreading fast across Asia, as the example of China made clear, and Truman feared that if South Korea fell, Japan would follow. Those fears drove his support for the war and Congress’s willingness to allocate 12 billion dollars to back Rhee’s resistance.

As the US ramped up production and braced for the unknown, Truman and the United Nations appointed General Douglas MacArthur to lead the combined US and UN forces. The support could not have come soon enough. The few UN forces that had reached Korea were still being hammered mercilessly, and had their lines collapsed, the army would have been split in half, forced to evacuate, and likely defeated barely two months into the war.

By August 1950, the tide began to turn. UN troops used their superior M4A3 Sherman and M26 tanks to dominate the battlefield, and a vastly superior air force to conduct reconnaissance and bomb the KPA’s supply lines. This stalled the North Korean advance—but it was not enough to break their lines or force a retreat.

The Incheon Landing: A Turning Point

Seeing the war nearly lost and recognizing that something had to change, MacArthur authorized a risky amphibious assault he hoped would give the US a far stronger foothold on the peninsula. He chose the KPA-occupied city of Incheon for several reasons. By landing 100 miles behind enemy lines, MacArthur hoped to completely sever the KPA’s supply lines and force them to fight on two fronts. Incheon also sat just southeast of Seoul, which he intended to retake quickly once the landing was secure.

Because the plan meant attacking an area notorious for harsh, unpredictable tides, Kim Il-Sung and his advisors did not believe Incheon was at risk of siege. They had left it undermanned and under-equipped, reallocating support to the frontlines instead. This proved one of the biggest blunders of the entire conflict.

On September 15th, 1950, nearly three months into the war, MacArthur landed on the occupied beaches of Incheon. Thanks to naval and air superiority—two things the KPA completely lacked—his forces quickly secured a landing zone and pushed inland. The operation was a major turning point, accomplishing everything MacArthur had hoped while dramatically lifting the morale of troops who had been fighting a losing war along the eastern coast for weeks.

The day after the landing, UN forces broke the line at Pusan, drove the KPA into full retreat, and linked up with MacArthur’s men at Osan. Ten days later, on September 25th, Seoul was recaptured, and Rhee’s government was reestablished.

China Enters the War

With Seoul secured, MacArthur received orders to advance further north and capture the North Korean capital of Pyongyang, left vulnerable by the sudden collapse of the KPA’s strategy. His mandate was to facilitate the complete destruction or dissolution of all KPA forces and unite the peninsula under a single government. Before the offensive continued, however, Truman reminded MacArthur that his orders were contingent on neither the Soviet Union nor China entering open conflict with US forces.

Acting on these orders, MacArthur sent his men past the 38th parallel on October 1st, and on October 18th they captured Pyongyang. He then demanded Kim Il-Sung’s unconditional surrender. By this point the KPA was in ruins, having suffered nearly 350,000 casualties—200,000 of them killed or wounded in battle, the rest captured. It had no air force, no navy, and its armored strength had been gutted.

After fleeing Pyongyang, this once-impressive force had been reduced to a mere 25,000, now dwarfed by the UN’s 230,000.

As soon as UN troops crossed the parallel, Kim began sending frantic appeals to China and the Soviet Union for reinforcements. Stalin, having already made clear the USSR would not engage directly, declined. Furious that the invasion had collapsed, he convened an emergency conference and condemned both the KPA and his own advisors for their failures.

The Yalu River Reversal

China, which had agreed before the war to send troops, now debated whether intervention was prudent. Its leaders disliked the idea of war with the United Nations, but Mao Zedong had been watching MacArthur close in on China’s border and was deeply unhappy about it. He stationed 260,000 troops from the People’s Liberation Army, or PLA, along the Chinese–North Korean border and lobbied the Soviet Union for air support. Stalin agreed to provide it, and after much internal debate—driven by fears that the US would continue advancing into China once North Korea had fallen—Beijing committed.

On October 19th, the day after Pyongyang fell, 300,000 Chinese troops crossed the Yalu River. On October 25th they made contact with the advancing US forces. The Chinese were ruthless and dominated the battlefield. They drove UN forces back from the border, rescued North Korea from certain defeat, pushed the fighting south of the 38th parallel, retook Pyongyang and then Seoul, and threatened to keep advancing until the UN was pushed off the peninsula entirely—and they had the manpower to make good on the threat.

MacArthur’s Atomic Gamble

Faced with this reversal, MacArthur made a recommendation that could have escalated the war and drastically changed the world. He proposed using nuclear bombs against the PLA stationed in North Korea as a show of force. His plan was to end the war by carpet-bombing the North with atomic weapons—demolishing and irradiating supply lines, cutting off further reinforcements, and trapping PLA forces between a wall of radiation and gunfire.

It was a merciless scheme that would have left countless dead and thousands of square miles uninhabitable. Yet for a man who had been battling in the Pacific for years and understood how brutal the fighting could become, MacArthur believed a quick end to the war was a merciful one—and many generals at the time agreed.

It may seem strange today, knowing how disastrous such an action could have been, but an important point bears noting. Throughout the Korean War, nuclear bombs were not yet as powerful as they would later become, and the concept of using them as a deterrent rather than a weapon had not yet been fully realized. Military leaders like MacArthur saw nuclear weapons as simply another tool in the arsenal, to be used in battle.

The idea of deliberately restricting one’s most powerful weapon for ethical reasons had never been done in human history. It had not yet occurred to commanders that this new weapon was fundamentally different from every other weapon mankind had created.

To this day, the only nuclear bombs ever dropped in battle remain the two used on Japan at Hiroshima and Nagasaki. Had MacArthur’s request been granted, it is likely that China would have formally declared war on the United States, and the Soviet Union would have been compelled to join on behalf of its allies. The result could have been a timeline in which the Cold War never happened and war between the UN and the Soviet Union erupted at a moment when both sides possessed nuclear weapons—and the US had already shown, twice, its willingness to use them.

What might have followed is anyone’s guess. Fortunately, cooler heads prevailed, and MacArthur’s request was denied.

Negotiations and an Unfinished War

Eventually, UN forces halted China’s advance with sustained bombing raids. But the disagreement over nuclear weapons opened a significant rift between US military leaders and the White House. MacArthur, irritated that his hands had been tied, began publicly criticizing Truman, calling the president’s unwillingness to use nuclear weapons “an enormous handicap, unprecedented in military history.”

Truman faced unthinkable decisions throughout his presidency, and among his admirable qualities was a refusal to back down from MacArthur’s criticism. After consulting his advisors and studying how past presidents such as Abraham Lincoln and James K. Polk had handled insubordination from their generals, he concluded that MacArthur’s conduct could not go unpunished. He relieved the general of command on April 11, 1951, replacing him with General Matthew Ridgway, who would lead all UN forces through the remainder of the war.

Over the next two years, from July 1951 to July 1953, UN forces and the PLA traded blows up and down the peninsula and along the 38th parallel. Battle lines shifted constantly, but neither side could secure a definitive advantage—the Chinese held superior numbers, while the Americans possessed more advanced technology. Once it became clear that neither side could win without catastrophic loss of life, an armistice was negotiated, establishing a demilitarized zone, or DMZ, along the border. The agreement was signed on July 27th, 1953, and the DMZ it created still exists today.

Unfortunately for the Korean people who remain divided, the armistice did not resolve the underlying issues between North and South. Neither side has ever been willing to admit defeat, and no official peace treaty has ever been signed. The Korean War is therefore still technically ongoing, with both sides continuing to prepare for future fighting. Occasional skirmishes have erupted along the DMZ, with gunfire sometimes exchanged, yet the two countries maintain a fragile coexistence under an agreement now more than 70 years old.

What Could Have Been

There remains much debate over whether the broader US leadership—setting MacArthur aside—ever seriously considered using nuclear weapons during the conflict, or whether threatening their use was merely a tool to dissuade Soviet intervention. Speaking publicly, Truman alluded to weighing all options, describing the use of nuclear weapons to end the war as under “active consideration.” He had also ordered a portion of the US nuclear arsenal transported to the Korean Peninsula and authorized its use under several extreme circumstances. Looking back, it certainly appears Truman knew the possibility of nuclear war was always on the horizon.

Yet, as historians have noted, Truman foresaw the dangers of using these weapons haphazardly and believed that dropping nuclear bombs on the peninsula would do more harm than good for the United States in the long run. He understood that such an action would brand him a warmonger on the world stage, hand China the propaganda it needed to rally further support, and risk dragging the Soviet Union into open conflict with the US against Stalin’s better judgment.

Still, it is frightening to consider how easily this conflict—and the many others that followed it throughout the Cold War—could have become the spark that plunged the US, the USSR, and the wider world into a war capable of killing millions, if not billions. In recent years, the Korean War has come to be called “The Forgotten War,” overshadowed by the conflicts that both preceded and followed it. But had things played out differently—had MacArthur had his way—it would have been a war impossible to forget: the world’s first, and not likely last, atomic war.