Even for an officer who had fought throughout the second world war, it was a hideous sight: dozens of shot-up trucks, heaped with the bleeding bodies of butchered American infantry, crawling south through the freezing mountains south of Kunu-Ri Pass, North Korea.
Major David Wilson of the Argyll and Sutherland Highlanders, a highly experienced veteran of the war against Japan, had never seen anything like it before. He was reminded of passages he had read of Elizabethan naval battles: 'I had never seen trucks with blood running out of the scuppers before,' he said.
The Highland major was unaware at the time - November 30, 1950 - that he was witnessing, at ground zero, a new superpower striding onto the world stage: China.
The American 2nd Infantry Division had been ground up in a Chinese roadblock of almost 10 kilometres. The virtual annihilation of two regiments in Kunu-Ri Pass, or 'The Gauntlet', plunged the United Nations Command, or UNC - the troops who had defended South Korea in July, then counter-invaded the North in October - into panic.
The longest retreat in American history, known as 'The Big Bug-out', got under way. Roads were clogged with traffic desperately pouring south. In fighting around Chosin Reservoir, deep in Korea's mountains, the US 1st Marine Division hacked its way through eight Chinese divisions surrounding it and escaped to the sea, but the US Army's 31st Regimental Combat Team was wiped out.
Pyongyang - the only communist capital captured by free-world troops in the cold war - fell. The UNC pulled back to South Korea. On New Year's Day, the Chinese charged across the 38th parallel. On January 4, Seoul fell.
The world was astounded. Over the previous two centuries, the 'sick man of Asia' had been humiliated by Britain in the opium wars; lost treaty ports and concessions on her coasts to Western powers; been trodden under by foreign troops during the Boxer Rebellion; been defeated in the Sino-Japanese war; and been the least effectual of the second world war allies.
Now, suddenly and shockingly, a new China had stood up. The Chinese People's Volunteers Army (CPVA) - actually, regular units of the People's Liberation Army - had saved communist North Korea from extinction. In the process, the most powerful military on earth had been humbled by a peasant army armed with nothing heavier than mortars.
This week, solemn ceremonies took place commemorating the outbreak on the Korean peninsula of one of the most apocalyptic episodes of the 20th century.
Colonised by the Japanese in 1910, Korea had been divided in an afterthought to the second world war, with the Soviets taking North Korea and the Americans the South. Efforts to hold unified elections failed; in 1948, separate states were established on both sides of the 38th parallel. Kim Il-sung, a former anti-Japanese guerilla and captain in the Red Army, took power in the North. Rhee Syngman, a nationalist who had sat out the Japanese occupation in the US, became president of the South.
On June 25, 1950, Kim launched a bold attempt for unification: his Soviet-trained and equipped army rolled south aiming to unify the peninsula under the red flag.
Ill-equipped southern troops collapsed. American-led UN troops intervened, but were pressed back towards the southwest port of Pusan. Battle raged as Kim's men tried to drive the Americans and southerners into the sea. But when, in September, General Douglas MacArthur, the US Supreme Commander in the Far East, unleashed his crack marines in an amphibious landing at Inchon, midway up the west coast, the North Koreans had no answer.
Remnants of Kim's army fled north with the UNC in hot pursuit. But as they approached the Sino-Korean border, resistance stiffened. Then - under grey skies heralding the approaching Manchurian winter, and out of the smoke of burning forests they had ignited as air cover - a new enemy struck.
Korean intervention was a risky move by Mao Zedong . He had barely completed his conquest of China and faced huge economic and social problems. Yet he sensed that advancing US troops posed a threat to his young republic; he compared the US to 'a tiger capable of eating human flesh'.
PLA leaders knew their inadequacies. Lin Biao , the senior military commander, declined to take command, citing illness. Instead it went to Peng Dehuai , a tough guerilla leader. Even Peng was worried: on being given command of troops destined for Korea, he was unable to sleep. His men had little transport, a mish-mash of small arms and virtually no heavy weapons. Lacking radios, they communicated by gongs and bugles. Yet his men had some advantages. They were tough peasants, used to darkness, rugged terrain, poor food. They were not road-bound, like the UNC; moreover, the UN forces were at the limit of their supply lines.
Using camouflage and concealment, CPVA troops infiltrated forward, then massed at the point of impact, at close range and usually under cover of darkness. When bugles sounded, the assault - known to terrified UNC troops as 'the human wave' - went in. If they could not overrun UNC units, Chinese soldiers - who referred to themselves as 'biped cavalry' for their cross-country speed - bypassed them and set road blocks and ambushes in their rear.
As his army crumbled, even MacArthur, one of the most egocentric commanders in history, recognised that events had spiralled beyond his control. 'We face an entirely new war,' he wrote.
The UNC would recover. The CPVA, its own logistical lines extended, was pushed back from Seoul. Mobile battles raged through spring. In summer 1951 truce negotiations began. The war that had almost become the third world war became the first 'limited war' as hill battles were fought for limited terrain gains, rather than total victory. In July 1953, an armistice was signed.
The disasters inflicted on the US by the CPVA in 1950 remain the worst defeats it has suffered since the second world war: no units were so badly cut up, even in the much longer Vietnam war. Likewise, the tragedy on the Imjin River in April 1951, when a British battalion was annihilated, remains Britain's worst military loss since 1945.
The Korean war was the first and only battlefield clash of superpowers. It marked the first extensive use of helicopters in combat and witnessed the first jet-versus-jet air combat. Yet the fighting on the ground was grinding and unglamorous, and atrocities were committed by both sides.
Unlike the second world war or Vietnam, Korea never captured the imagination of novelists or filmmakers, to the point where few people in the West know much - if anything - about it. Its legacy is mixed.
'People in the West need to know two key points about the Korean war: first, it was the result of the US and Soviet division of Korea, and of the cold war gamesmanship those two then played,' said Charles Hanley, Pulitzer prize-winning co-author of The Bridge at No Gun Ri, an investigation into a massacre by US troops. 'Secondly, an estimated minimum of two million innocent civilians in North and South Korea were killed in the war, many of them by American bombing and gunfire.'
Moreover, the war ended with a truce, not a peace treaty. Sixty years on, as the recent sinking of the South Korean warship Cheonan shows, peace is no closer, making the war the longest-running conflict on earth.
UNC veterans are bitter they did not reunify Korea, but those who visit the South are stunned at its transformation: a brutal regime, a broken land and a cowed populace have become a dynamic economy, a thriving democracy and a vibrant society.
Chinese veterans saved North Korea - then, as now, a strategic buffer on China's northeastern flank. But what they feel when, after visiting the Korean war museum in the border city of Dandong , Liaoning , they gaze over the Yalu River bridges at the decrepit state they defended is another matter.
Today's China has grown the economic muscle to match the military sinews she flexed so astonishingly in 1950. With the Middle Kingdom increasingly integrating with the outside world in the economic sphere, and increasingly adopting global norms, the question must be asked: for how long will 21st-century Beijing tolerate the provocations, inadequacies and unwillingness to reform of the regime her soldiers saved when they crossed the Yalu 60 years ago?