Since 1904, Japan had effectively occupied Korea. In the final stages of World War II, an agreement was reached between the United States and the Soviet Union: the Soviets would occupy South Korea only up to the 38th parallel. The United States forces that arrived in Korea were ill-prepared for their duties, lacking knowledge of Korea’s history and its relationship with Japan. For many Koreans, independence and unification were their primary objectives.

After considerable deliberation, the United States supported Syngman Rhee, a Korean nationalist who had been exiled to the United States in 1907. The United States requested that the United Nations resolve the issue of a divided Korea. Despite Soviet objections, a United Nations commission approved elections in Korea. However, the communists in the South boycotted the election, preventing its participation in the North. In the South, conservative parties allied with Rhee secured a majority of the vote in an election in which 80% of eligible Korean voters participated. Rhee was subsequently elected President of the newly-declared independent South Korea in October 1948. In contrast, the Soviets installed Kim II Song as the leader of the North.

As the United States reduced its military presence in the post-war era, the American garrison of 40,000 rapidly dwindled to a force of 472 officers and personnel, comprising the Korean Military Advisory Group (KMAG). The Korean People’s Army (ROK), commonly referred to as the Korean Army, was provided with only light weaponry. In contrast, the North Korean Army was heavily equipped with tanks and other armored vehicles. The communist victory in China, coupled with the inaugural Soviet nuclear tests conducted in 1949, prompted a new United States policy of containment in Asia. This policy, formally known as NSC 48/2, advocated for the containment of communism primarily through non-military means, while simultaneously providing economic and military assistance to non-communist regimes in the region.

On January 5, 1950, Secretary of State Dean Acheson delivered a speech at the National Press Club, articulating the American policy. He outlined the countries that the United States would defend with force: Japan, the Ryukyu Islands, and the Philippine Islands. Notably, Korea was not included in this list. The withdrawal of the remaining American forces from Korea, coupled with North Korean leader Kim Jong-il’s belief that the United States would refrain from intervening, convinced the North Koreans to attempt to unify the country through force. The Soviet Union, led by Joseph Stalin, and the People’s Republic of China, led by Mao Zedong, both concurred with Kim Jong-il’s assessment of the United States and his intentions to unify the country by force. In June 1950, this conflict erupted.