U.S. Marines stand in a trench ready to repel an expected communist Chinese attack on their position during the Korean War, May 1, 1951.

The United States left behind a military force to take the U.S. Army's place in protecting the South Korean state, but deliberately had armed it with only light arms to discourage the South Korean government from invading the North. In retrospect, this probably was a grave blunder, because it made South Korea vulnerable to North Korea's Soviet-made tanks and heavy weapons.

The North's leader, Kim Il Sung, begged Josef Stalin for permission to invade the South and eventually got it. In June 1950, the North Koreans launched a surprise invasion. In a radio address, Kim Il Sung denounced the South's "puppet government" and vowed to "execute traitors everywhere," including South Korea's elected president, Syngman Rhee.

If Kim hoped for a quick victory, he was tragically mistaken. The South Korean defenders and the remnant of U.S. occupation troops were quickly pushed back to the southeastern corner of the peninsula, but U.S. Gen. Walton "Stand or Die" Walker and his men held off the invaders for six weeks, buying crucial time. During that interval, the United Nations Security Council passed a resolution calling on all members to repel the aggression, which the Soviet representative could not veto because he'd previously walked out in protest. An international force of 300,000 troops — 260,000 of them Americans — was amassed.

In September 1950, Gen. Douglas MacArthur staged a daring counterattack, seizing the fortified port city of Inchon, well behind enemy lines. The North Koreans fled in confusion. At the behest of President Harry Truman, who wanted to unify Korea, MacArthur advanced into North Korea and seized the North Korean capital of Pyongyang. But when, against White House orders, he continued chasing the retreating North Koreans across the Chinese border, the communist Chinese Army jumped into the conflict.

The U.N. and communist forces hunkered down into a bloody, two-year-long stalemate along the 38th parallel. Finally, in July 1953, with Josef Stalin dead and the new U.S. president, Dwight Eisenhower, mulling over whether to use atomic bombs to end the conflict, an armistice ending hostilities was signed.

Kim Il Sung managed to avoid responsibility for the disastrous war, which resulted in an estimated half-million North Korean casualties. Instead, he turned it to his advantage, blaming battlefield setbacks on a potential political rival, People's Army Gen. Mu Jong, who was imprisoned and never heard from again. Kim purged scores of other party members as well. In particular, he eliminated the southern intellectuals, artists and academics who — like their counterparts in Hollywood and Greenwich Village in 1930s America — mistakenly had seen communism as a way to right injustices.

Power became more concentrated in the hands of Kim and a small cadre of trusted hard-liners who'd been with him as guerillas in Manchuria. That leadership had little education or exposure to the outside world except for Soviet and Maoist ideology and readily embraced Kim's deeply suspicious world view.