How Korea Was Divided

For over 35 years (1910–1945), the Korean Peninsula was subjected to brutal colonial occupation by the Empire of Japan. Korean culture was suppressed, language was banned, and thousands were conscripted into forced labor. As World War II entered its final weeks in the summer of 1945, the Allied powers faced the urgent question of how to disarm Japanese forces and stabilize the peninsula.

The decision they made in haste would permanently shatter Korea into two hostile nations, setting the stage for one of the most persistent conflicts in modern history.

The Cairo and Potsdam Conferences

The framework for Korea's postwar future was discussed at the Cairo Conference (1943). US President Franklin D. Roosevelt, British Prime Minister Winston Churchill, and Chinese Generalissimo Chiang Kai-shek signed the Cairo Declaration, which declared that "in due course, Korea shall become free and independent."

This vague phrase "in due course" reflected the Allies' belief that Korea was not ready for immediate self-rule and should be managed under a multinational trusteeship. Later, at the Potsdam Conference (1945), the Soviet Union formally agreed to this framework after declaring war on Japan.

The 30-Minute Boundary: The 38th Parallel

In August 1945, following the atomic bombings of Hiroshima and Nagasaki, Soviet troops entered northern Korea. Fearing that the Red Army would occupy the entire peninsula before US forces could arrive, the US State-War-Navy Coordinating Committee scrambled to draft a division proposal.

On the night of August 10, 1945, two young officers—Dean Rusk (later Secretary of State) and Charles Bonesteel—were given just 30 minutes to define an American occupation zone.

• Armed only with a 1936 National Geographic map of Asia, they noticed the 38th parallel north.
• They chose this line because it divided the peninsula roughly in half, but crucially kept the capital, Seoul, inside the American zone.
• Although Rusk admitted the line was arbitrary and unfavorable if the Soviets rejected it, Soviet Premier Joseph Stalin accepted the proposal without objection, halting his troops at the line.

The Hardening of the Border (1945-1948)

What was intended as a temporary military demarcation line quickly solidified due to the emerging Cold War:

• The Soviet Zone (North): The Soviets established a communist administration, building up local committees led by Kim Il Sung, a former anti-Japanese guerrilla officer in the Soviet Red Army. In September 1948, the Democratic People's Republic of Korea (DPRK) was proclaimed.
• The American Zone (South): The US military government backed Syngman Rhee, an elite anti-communist politician who had spent decades in exile. In August 1948, the Republic of Korea (ROK) was established.

Both leaders claimed sole legitimacy over the entire peninsula and were committed to unifying it by force.

The Korean War and the DMZ (1950–Present)

On June 25, 1950, Kim Il Sung launched a massive, Soviet-authorized surprise invasion of the South. The conflict quickly became a global proxy war. The United Nations Command, led by Harry Truman's administration, deployed troops to support the South, while the newly established People's Republic of China, authorized by Mao Zedong and supported by Soviet air power, intervened to rescue the North.

After three years of devastating warfare that claimed an estimated 3 million lives, the battle lines settled near the original border. On July 27, 1953, the Korean Armistice Agreement was signed at Panmunjom by the UN Command, the North Korean People's Army, and the Chinese People's Volunteers. Syngman Rhee refused to sign, protesting the failure to unify the country.

• The armistice established the Military Demarcation Line (MDL) surrounded by the Demilitarized Zone (DMZ), a 4-km-wide, 250-km-long buffer strip.
• Today, the DMZ remains the most heavily fortified border in the world. Because no peace treaty was ever signed, North and South Korea remain technically at war, leaving thousands of families permanently separated.