On June 25, 1950, North Korea’s invasion of South Korea ignited a conventional war that had origins dating from at least the end of World War II. In April 1945, President Harry S. Truman abandoned a trusteeship plan for postwar Korea in favor of seeking unilateral U.S. occupation of the peninsula after an atomic attack forced Japan’s prompt surrender. Soviet entry into the Pacific war led to a last minute agreement dividing Korea at the 38th parallel into zones of occupation. Two Koreas emerged after Soviet-American negotiations failed to agree on a plan to end the division. Kim Il Sung in the north and Syngman Rhee in the south both were determined to reunite Korea, instigating major military clashes at the parallel in the summer of 1949. Moscow and Washington opposed their clients’ invasion plans until April 1950 when Kim persuaded Soviet Premier Joseph Stalin that with mass support in South Korea, he would achieve a quick victory. At first, Truman hoped that South Korea could defend itself with more military equipment and U.S. air support. Commitment of U.S. ground forces came after General Douglas MacArthur, U.S. occupation commander in Japan, visited the front and advised that the South Koreans could not halt the advance. Overconfident U.S. soldiers would sustain defeat as well, retreating to the Pusan Perimeter, a rectangular area in the southeast corner of the peninsula. On September 15, MacArthur staged a risky amphibious landing at Inchon behind enemy lines that sent Communist forces fleeing back into North Korea. The People’s Republic of China viewed the U.S. offensive for reunification that followed as a threat to its security and prestige. In late November, Chinese “volunteers” attacked in mass. After a chaotic retreat, U.S. forces counterattacked in February 1951 and moved the line of battle just north of the parallel. After two Chinese offensives failed, negotiations to end the war began in July 1951, but stalemated in May 1952 over the issue of repatriation of prisoners of war. Peace came because of Stalin’s death in March 1953, rather than President Dwight D. Eisenhower’s veiled threat to stage nuclear strikes against China. Scholars have disagreed about many issues surrounding the Korean War, but the most important debate continues to center on whether the conflict had international or domestic origins. Initially, historians relied mainly on U.S. government publications to write accounts that ignored events prior to North Korea’s attack, endorsing an orthodox interpretation assigning blame to the Soviet Union and applauding the U.S. response. Declassification of U.S. government documents and presidential papers during the 1970s led to the publication of studies assigning considerable responsibility to the United States for helping to create a kind of war in Korea before June 1950. Moreover, left revisionist writers labeled the conflict a classic civil war. Release of Chinese and Soviet sources after 1989 established that Stalin and Chinese leader Mao Zedong approved the North Korean invasion, prompting right revisionist scholars to reassert key orthodox arguments. This essay describes how and why recent access to Communist documents has not settled the disagreements among historians about the causes, course, and consequences of the Korean War.