Korea was now permanently two separate nations. Meanwhile, in 1949, Communists won a civil war in China and seized control of the country’s government. President Truman and his advisers feared that Communism was spreading.
So when North Korea attacked South Korea and occupied its capital, Seoul, in 1950, Truman believed he had to take a stand. He turned to the United Nations (U.N.), which authorized its members to fight back the invaders. (Although 16 U.N. countries sent troops, most were from the U.S.)
Early on, the U.N. forces seemed poised for victory. After retaking Seoul in September 1950, they began making their way across the 38th parallel, toward North Korea’s border with China.
But in late November of that year, some 300,000 Chinese troops swept south to aid North Korea. Outnumbered, the U.N. forces were soon in full retreat.