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 shared maritime boundary with Japan.

For much of the early twentieth century, Korea was a protectorate of Japan. Japan had seized control of the peninsula—which had been ruled for centuries by the Joseon 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. At the same time, however, it faced growing economic sanctions from the United States and its Western allies, which were worried about Japan’s 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 and drafted more than 240,000 men into the military as both soldiers and civilian employees. More than 5 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. The most notorious abuse, though, 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. To this day, Japan contests its responsibility for the treatment of these women.

As World War II ground on, the developments that would shape Korea’s postwar trajectory began to unfold. On November 27, 1943, U.S. President Franklin D. Roosevelt, British Prime Minister Winston Churchill, and Chiang Kai-shek, the leader of China’s Nationalist Party, met in Cairo, Egypt. These allies decided that after Japan’s surrender, the country would be stripped of the territories it had acquired since beginning its expansionist campaign in the late nineteenth century—including Korea, which the allies acknowledged had been a victim of Japanese aggression. The resulting declaration noted that the allies, “mindful of the enslavement of the people of Korea, are determined that in due course Korea shall become free and independent.” 

By early 1945, the war in the Pacific was entering its final phase. At the Yalta Conference in Crimea, in the Soviet Union, in February, Roosevelt and Soviet leader Joseph Stalin 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 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 two atomic bombs—the only ones 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 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 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 the north for 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, creating both administrative bodies and a North Korean Workers’ Party that united various left-wing groups. At the same time, Soviet-encouraged land reform—which redistributed land from Japanese and Korean landowners to poor farmers—drove thousands of former landowners and Japanese collaborators into the southern part of the peninsula. 

The issue was brought before the UN General Assembly in September 1947. World leaders had established the United Nations just two years earlier. It was created with their collective hopes of preventing a third world war. The Korea issue was an early test of the organization’s ability to manage decolonization efforts and great power disputes of the postwar era.

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 created the UN Temporary Commission on Korea (UNTCOK) to 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. The commission was also charged with observing and reporting threats to and violations of the boundary at the thirty-eighth parallel.

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 (ROK), 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 UNTCOK. Four days after the proclamation of the Republic of Korea, authorities in the north cut power transmission to the south, reinforcing the peninsula’s division. Less than a month later, the Democratic People’s Republic of Korea (DPRK), the official name of North Korea, was born, and its capital established as 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. In December, the UN General Assembly passed Resolution 195, recognizing the ROK as the “lawful” Korean government. But the Soviet Union again disagreed, recognizing the DPRK instead.

The United States kept military forces in South Korea until 1949, but its support for Rhee’s government was half-hearted. 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 the spring of 1950, however, he relented and approved Kim’s plan. Kim also sought support—at Stalin’s encouragement—from 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 immediately brought the matter to the attention of the UN Security Council. The Council met the next day. UN Secretary-General Lie began the meeting with a report from the UNTCOK. The report claimed that the situation was “assuming the character of full-scale warfare” and he urged Security Council members to take action. U.S. representative Ernest A. Gross then introduced a draft resolution stating that the invasion was a breach of the peace in violation of the UN Charter. This resolution, known as Resolution 82, passed with a vote of nine to zero, with one abstention. Resolution 83, passed two days later, 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 country had been boycotting Security Council meetings since January, after the defeat of its proposal to replace the nationalist Chinese delegation holding a permanent seat with a delegation from Mao’s Communist China. 

Within days of the passage of Resolution 83, naval units and aircraft supplied by several nations, as well as the first major formations of U.S. troops, were making their way to South Korea. 

On July 7, the Security Council adopted Resolution 84. It called for a unified UN command and put the United States in charge 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 80,000 U.S. troops were in Korea. They were fighting alongside some 90,000 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. 

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 onto land. This plan echoed the D-Day landings on the beaches of Normandy, 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. This was something the United Nations had envisioned since the surrender of Japan. But realizing this opportunity would require invading North Korea, which could bring UN member states—particularly the United States and the Soviet Union—in conflict with one another.