MacArthur told Washington China wouldn't intervene. Then 300,000 Chinese soldiers silently crossed the border in the dark, and the war the US thought it had won vanished overnight.
As UN forces swept north after the Inchon triumph, pushing toward the Chinese border at the Yalu River, warnings multiplied that China would intervene. China's foreign minister declared through diplomatic channels that China 'cannot sit idly by.' MacArthur dismissed the warnings, telling President Truman personally in October 1950 that Chinese intervention was unlikely and, if it occurred, would be 'the greatest slaughter' for China's forces.
Mao Zedong had been deliberating since early October. He feared that American forces on China's border represented an existential threat — that the US would use Korea as a platform to attack China itself. The decision was agonizing: China's military was exhausted from years of civil war, had no air force capable of challenging American airpower, and had no nuclear weapons. Mao went forward anyway.
The Chinese People's Volunteer Army — so named to avoid a formal declaration of war between China and the United States — crossed the Yalu River on October 19, 1950. Approximately 200,000 troops initially entered Korea; by late November the force would swell to 300,000. They marched only at night, hiding under forest cover by day, avoiding roads that aircraft could observe. UN air reconnaissance missed them almost entirely.
The Chinese struck first on October 25, hitting South Korean units. They fought for two weeks, then abruptly disappeared — a deliberate tactic. MacArthur and his intelligence staff interpreted the withdrawal as evidence the Chinese had suffered heavy losses and retreated. In fact, they were regrouping, waiting, and bringing in more forces. It was a trap.
On the night of November 25-26, the trap closed. An estimated 300,000 Chinese soldiers attacked UN forces stretched thin across northern Korea. Bugles, whistles, and cymbals blared through the frozen darkness as human waves overran American and South Korean positions. The 2nd Infantry Division was nearly destroyed in a single night; the survivors suffered 'the longest retreat of an American unit in history.'
China's intervention transformed the war fundamentally. What had appeared to be a UN victory — the imminent unification of Korea under a pro-Western government — evaporated. UN forces retreated hundreds of miles south, Seoul fell again, and a war that seemed almost over stretched into two more years of grinding stalemate. The geopolitical consequences were equally vast: China had demonstrated that it could confront and bloody the most powerful military on earth.