The main objective of the first South Korean government was the suppression of leftist groups, some of them independent but many supported by the Communist government of North Korea. The United States, concerned about leftist guerrilla activity and the potential of invasion from North Korea, delayed withdrawing its occupation forces in South Korea until June 1949.

However, the security situation remained extremely tenuous in the Korea Peninsula. The North Korean leader, Kim Il Sung, sought to unify the Korea Peninsula under Communist rule. In June 1950 he launched a full-scale military invasion of South Korea, thereby starting the Korean War. The United States immediately gained UN Security Council support for the defense of South Korea and committed American ground troops to the war. The Korean War was ultimately one of the most destructive and deadly wars of the 20th century. Perhaps as many as 4 million Koreans died throughout the peninsula

During the war, South Korean president Rhee governed under martial law, and he used his power to force the legislature to adopt a constitutional amendment providing for popular election of the president. Rhee was popularly elected to a second term in 1952.

In July 1953 an armistice agreement signed by the UN, North Korea, and China—South Korea refused to sign—ended the fighting of the Korean War. Without a formal peace treaty, however, North Korea and South Korea technically remained at war. Their shared border, known as the Demilitarized Zone (DMZ), remained heavily fortified and guarded on both sides. With the consent of South Korea, the United States continued to maintain a military presence in the country.

South Korea made a slow recovery from the war. Rhee was unable to produce any significant economic development despite much aid from the United States. He easily won reelection in 1956 and 1960, but blatant manipulation of the 1960 elections led to nationwide protests that culminated in Rhee’s forced resignation on April 27, 1960. The moderate government of John M. Chang that followed Rhee’s departure implemented liberalizing reforms in many areas, but economic development still lagged. Military leaders, fearing growing instability and wary of student agitation for talks with North Korea, staged a coup on May 16, 1961.