Background

Slightly smaller than the United Kingdom, the Korean Peninsula juts southward from China between the Yellow Sea and the Sea of Japan. The peninsula’s northern edge shares a long border with China and a short one with Russia. East of the peninsula is a maritime boundary with Japan.

For much of the early twentieth century, Korea was a protectorate of Japan. Japan seized control of the peninsula—which had been ruled for centuries by the Choson dynasty—in the First Sino-Japanese War (1894–95). Japan maintained its control despite challenges from Russia and frequent domestic uprisings. Japan’s grip on the peninsula was tight politically, economically, and culturally. Japanese authorities outlawed social and political organizations. They also banned the teaching of the Korean language, and forced the population to speak Japanese and adopt Japanese names. Japan did industrialize the peninsula, building highways, railroads, and factories. However, much of this effort was gradually directed toward military use by an increasingly aggressive imperial Japan.

By the late 1930s, Japan was well into its campaign of conquest throughout East Asia. In 1931, it invaded and seized Manchuria, a resource-rich region in northeast China, and renamed it Manchukuo. It followed up in 1937 with a full-scale invasion of China, starting the Second Sino-Japanese War. In 1936 and 1937, Japan formalized its friendship with Nazi Germany and Fascist Italy by signing the Anti-Comintern Pact. This created the alliance that would come to be known as the Axis powers. 

When war broke out in Europe in 1939, Japan saw an opportunity to seize European colonies and grow its empire. However, it faced growing economic sanctions from the United States and its Western allies, who were worried about Japan’s imperial expansion. On December 7, 1941, the Japanese carried out a surprise military strike on the U.S. naval base at Pearl Harbor in Hawaii. The attack was intended to avoid an extended conflict by debilitating the U.S. fleet. Instead it brought the United States into World War II. 

During the war, Japan brutally exploited Korea’s people. It drafted more than 240,000 men into the military, as soldiers and civilian employees. More than five million men and women were conscripted to work in war-related industries under dangerous conditions. In addition, some 670,000 were forcibly brought to Japan. Hundreds of thousands died. But the most notorious abuse involved the so-called comfort women. Up to two hundred thousand Korean women were kidnapped to serve as sex slaves in military brothels, or comfort stations, for Japanese soldiers. Japan contests its responsibility for the treatment of these women to this day.

By early 1945, the war in the Pacific was entering its final phase. U.S. leaders started to think about how to handle the territories Japan had acquired since beginning its expansionist campaign in the late nineteenth century. At the Yalta conference in Crimea in February, U.S. President Franklin D. Roosevelt and Soviet leader Joseph Stalin, who were allies against Nazi Germany in the war in Europe, agreed to establish an international trusteeship for Korea following Japanese surrender. This agreement, though, was only a general framework. The precise arrangements for governing a postwar Korea were not finalized.

The Soviet Union declared war on Japan on August 8, 1945. Shortly thereafter, Soviet forces invaded Korea and Manchuria. U.S. military leaders feared that, if the Soviet Union ended up occupying Korea in the course of the conflict, it would never cede control. This concern spurred the U.S. government to finalize a formula for administering the peninsula. U.S. officials hurriedly proposed the thirty-eighth parallel as a demarcation line between U.S. and Soviet forces, cutting the peninsula almost in half. This line was intended as a temporary operational boundary that would hinder Soviet ambitions and prevent confusion among military forces operating in Korea.

The order came down on August 17, 1945, two days after Japan’s surrender ended World War II. (Japan had surrendered days after the United States dropped atomic bombs—the only two ever used in combat—on Hiroshima and Nagasaki, devastating the two cities and killing some 170,000 people.) The Soviet Union would accept the Japanese surrender of weapons and troops and take control of the territory north of the thirty-eighth parallel. South of the parallel, these tasks would fall to the United States. 

Temporary peace was thus brought to the Korean Peninsula after the end of the war. The political and economic situation remained complex and unstable. U.S. troops landed in Korea on September 8, 1945, following the Soviet troops, who had arrived the previous month. Because Korea had been a Japanese colony since 1910, no Korean government was in place ready to reclaim authority. The political scene was fragmented as various leaders jostled for power. One of these was Kim Il-sung, a military leader with close connections to the Soviet Union who would become the first leader of North Korea. Other contenders were ultranationalists oriented more toward the West. This camp included. Syngman Rhee, who had earned a doctorate at Princeton University and, with U.S. backing, would become the first leader of South Korea.

Korea’s economy, meanwhile, was suffering. When the Japanese departed after World War II, many companies were left without managers, capital, and other resources. This led to unemployment and shortages of vital goods. The peninsula’s division at the thirty-eighth parallel did not help. Korea’s heavy industry and energy production were concentrated in the north. This left the south’s primarily agrarian economy dependent on electricity transmission and transport of other essentials. This, along with the return of millions of Koreans from elsewhere in the region, caused significant social unrest and protests against the U.S. military government in the south.

At first, the United States and the Soviet Union shared a vision for a united, independent Korea. The foreign ministers of the World War II allies, meeting in December 1945 in Moscow, agreed that Korea would have a five-year trusteeship. Attempts to fulfill this vision stumbled. A joint U.S.-Soviet commission met occasionally in 1946 and 1947 but was unable to establish a Korean government because the United States and Soviet Union disagreed on who should participate. 

Meanwhile, the division on the peninsula remained stark. The south was disorganized, afflicted by economic instability, political differences among Koreans, and uncertain U.S. policy. In the north, by contrast, the Soviet Union smartly consolidated control. It created administrative bodies and a North Korean Workers’ Party that united various left-wing groups. At the same time, Soviet-encouraged land reform. This redistributed land from Japanese and Korean landowners to poor farmers. At the same time, it drove thousands of former landowners and Japanese collaborators into the southern part of the peninsula. 

Failing to make progress with Moscow on an acceptable path for Korean independence, the Truman administration took the issue to the UN General Assembly in September 1947. World leaders had established the United Nations just two years earlier, vesting in it their collective hopes of preventing a third world war. In the General Assembly, the United States continued to advance the vision of a united, independent peninsula. In November 1947, the General Assembly adopted UN Security Council Resolution 112, which asserted “that the national independence of Korea should be re-established and all occupying forces then withdrawn at the earliest possible date.” To facilitate this plan, the resolution established the UN Temporary Commission on Korea. This resolution would pave the way for elections for a national assembly. This assembly was to set up a government that would assume full administrative responsibilities and work with the United States and Soviet Union to clear Korea of occupying troops.

Despite the commission, the Korean Peninsula moved no closer to unified governance. Instead, politics continued to evolve separately on either side of the thirty-eighth parallel. Rhee, by now an influential, U.S.-backed nationalist leader, favored independence as soon as possible—even though a declaration of independence would have effect only in the south as long as the United States and the Soviet Union remained at loggerheads. In a UN-supervised election in May 1948 for a constitutional assembly in the south, Rhee came out in front. Under his leadership, the assembly adopted a constitution outlining a presidential system of government. The Republic of Korea, which remains the official name of South Korea, was proclaimed on August 15, 1948. Rhee took office in Seoul as its first president.

North of the thirty-eighth parallel, the Soviet Union refused to admit the Temporary Commission. Four days after the proclamation of the Republic of Korea, authorities in the north cut power transmission to the south, further reinforcing the peninsula’s division. Less than a month later, the Democratic People’s Republic of Korea, the official name of North Korea, was born, its capital in Pyongyang. Its first leader, Kim Il-sung, claimed jurisdiction over the entire peninsula. By the end of 1948, he had solidified control over the north’s administrative structures, military forces, and Communist Party.

The United States kept military forces in South Korea until 1949, but its support for Rhee’s government was half-hearted. U.S. officials were unsure of Seoul’s political future and doubted its strategic value. The United States declined to commit to defending South Korea even though it had made such commitments to Japan and the members of the new North Atlantic Treaty Organization (NATO) in the immediate post–World War II years. (In a speech on January 12, 1950, Secretary of State Dean Acheson did not include Korea in what he described as a U.S. defensive perimeter.) The South Korean government established its own army in September 1948. However, a rebellion by some army units the following month, and a purge thereafter, left the force weak. By 1950, it had fewer than one hundred thousand soldiers and lacked advanced equipment such as tanks, heavy weapons, and combat aircraft.

The contrast with North Korea was stark. Although the Soviet Union had withdrawn its forces in late 1948, focusing instead on increasing its control in Eastern Europe, it continued to provide training and arms to North Korean forces. With this assistance, by mid-1950 Pyongyang had built up a force of 150,000 to 200,000 troops. The North Korean People’s Army had fearsome Soviet weapons, including tanks and fighter planes, at its disposal.

Fueled by his military might and ambitions to control the entire peninsula, Kim Il-sung sought Stalin’s support for an invasion of the south. At first, Stalin resisted. In spring 1950, however, he relented and approved Kim’s plan. Kim also sought support—at Stalin’s encouragement—from the Chinese leader Mao Zedong, who had recently emerged victorious in the Chinese civil war and successfully pushed nationalist forces offshore to Taiwan. Early on the morning of June 25, 1950, North Korean soldiers crossed the thirty-eighth parallel. The Korean War began. The main offensive was aimed at the South Korean capital, Seoul, which fell in only three days. 

The United States, under Truman, responded both diplomatically and militarily. On the diplomatic front, it immediately secured UN Security Council Resolution 82, calling for the invasion to halt. Resolution 83, passed two days later, on June 27, called on UN member states to provide “such assistance to the Republic of Korea as may be necessary.” The Soviet Union, as a permanent member of the Security Council, could have vetoed these resolutions. However, the Soviet Union had been boycotting Security Council meetings after the defeat of its proposal to replace the nationalist Chinese delegation holding the council’s permanent seat with a delegation from Mao’s communist China. 

Truman initially gave military equipment to the South Korean army. Truman also deployed U.S. aircraft and ships. As South Korea’s military faltered, he allowed limited U.S. ground troops to enter the fight in early July. On July 7, UN Security Council Resolution 84 put the United States in command of all military operations to assist the South Koreans. Under MacArthur’s command, the UN mission initially struggled. The disorganized, ill-equipped South Korean army was no match for the invading North Koreans. U.S. personnel, still recovering from World War II, had to contend with equipment shortages, refugees fleeing the fighting, and even a lack of water. 

By the end of the summer, about two months after the war began, more than eighty thousand U.S. troops were in Korea. They were fighting alongside some ninety thousand South Korean troops and a 1,600-man British contingent. These allied forces held only the Pusan perimeter in southeastern South Korea—North Korean soldiers had overrun the rest of the peninsula. Communist control, which the United States had sought to limit since the closing months of World War II, now threatened all of Korea. 

To reverse the turn of events, MacArthur was planning a risky landing of forces at Inchon, an area on the west coast of South Korea near Seoul. He aimed to cut off enemy supply lines behind the North Korean troops, which had advanced farther south. The aim was to divide the enemy forces and break their hold on Seoul, enabling South Korea’s government to eventually regain control. The Inchon landing would be an amphibious assault: troops would arrive by sea and proceed on to land. This plan echoed the D-Day landings on the beaches of Normandy, in France, on June 6, 1944. Those assaults were costly but spectacularly successful and turned the tide against Nazi Germany in the European theater of World War II.

Should the Inchon landing succeed, the liberation of South Korea would be at hand. With that could come an opportunity to unify the peninsula, as U.S. leaders had envisioned since the surrender of Japan. But realizing this opportunity would require invading North Korea, which could bring the United States into conflict with the Soviet Union, China, or both.