North and South Korea were first unified by the Silla Dynasty in the seventh century CE, and were unified for centuries under the Joseon Dynasty (1392–1910); they share the same language and essential culture. Yet for the last six decades and more, they have been divided along a fortified ​demilitarized zone (DMZ). That division took place as the Japanese empire crumbled at the end of World War II, and the Americans and Russians quickly divided up what remained.

Key Takeaways: The Division of North and South Korea Despite being unified off and on for nearly 1,500 years, the Korean peninsula was divided into North and South as a result of the breakup of the Japanese empire at the end of World War II.

The precise location of the division, at the 38th parallel latitude, was chosen by lower-level U.S. diplomatic personnel on an ad hoc basis in 1945. At the end of the Korean War, the 38th parallel became a demilitarized zone in Korea, an armed and electrified barrier to traffic between the two countries.

Reunification efforts have been discussed many times since 1945, but they are seemingly blocked by steep ideological and cultural differences developed since that time.

Korea After World War II

This story begins with the Japanese conquest of Korea at the end of the 19th century. The Empire of Japan formally annexed the Korean Peninsula in 1910. It had run the country through puppet emperors since its 1895 victory in the First Sino-Japanese War. Thus, from 1910 until 1945, Korea was a Japanese colony.

As World War II drew to a close in 1945, it became clear to the Allied Powers that they would have to take over the administration of Japan's occupied territories, including Korea, until elections could be organized and local governments set up. The U.S. government knew that it would administer the Philippines as well as Japan itself, so it was reluctant to also take trusteeship of Korea. Unfortunately, Korea just wasn't a very high priority for the U.S. The Soviets, on the other hand, were more than willing to step in and take control of lands that the Tsar's government had relinquished its claim to after the Russo-Japanese War (1904–05).

On Aug. 6, 1945, the United States dropped an atomic bomb on Hiroshima, Japan. Two days later, the Soviet Union declared war on Japan and invaded Manchuria. Soviet amphibious troops also landed at three points along the coast of northern Korea. On Aug. 15, after the atomic bombing of Nagasaki, Emperor Hirohito announced Japan's surrender, ending World War II.

The U.S. Splits Korea Into Two Territories

Just five days before Japan surrendered, U.S. officials Dean Rusk and Charles Bonesteel were given the task of delineating the U.S. occupation zone in East Asia. Without consulting any Koreans, they arbitrarily decided to cut Korea roughly in half along the 38th parallel of latitude, ensuring that the capital city of Seoul—the largest city in the peninsula—would be in the American section. Rusk and Bonesteel's choice was enshrined in General Order No. 1, America's guidelines for administering Japan in the aftermath of the war.

Japanese forces in northern Korea surrendered to the Soviets, while those in southern Korea surrendered to the Americans. Although South Korean political parties quickly formed and put forward their own candidates and plans for forming a government in Seoul, the U.S. Military Administration feared the leftist tendencies of many of the nominees. The trust administrators from the U.S. and the USSR were supposed to arrange for nation-]wide elections to reunify Korea in 1948, but neither side trusted the other. The U.S. wanted the entire peninsula to be democratic and capitalist while the Soviets wanted it all to be communist.

North and South Korea, Divided at the 38th Parallel. US Central Intelligence Agency

Impact of the 38th Parallel

At the end of the war, Koreans were united in joy and hope that they were going to be a single independent country. The establishment of the division—made without their input, let alone their consent—eventually dashed those hopes.

Further, the location of the 38th Parallel was in a bad place, crippling the economy on both sides. Most heavy industrial and electrical resources were concentrated north of the line, and most light industrial and agricultural resources were to the south. Both North and South had to recover, but they would do so under different political structures.

At the end of WWII, the U.S. essentially appointed the anti-communist leader Syngman Rhee to rule South Korea. The South declared itself a nation in May 1948. Rhee was formally installed as the first president in August and immediately began waging a low-level war against communists and other leftists south of the 38th parallel.

Meanwhile, in North Korea, the Soviets appointed Kim Il-sung, who had served during the war as a major in the Soviet Red Army, as the new leader of their occupation zone. He officially took office on Sept. 9, 1948. Kim began to quash political opposition, particularly from capitalists, and also began to construct his cult of personality. By 1949, statues of Kim Il-sung were springing up all over North Korea, and he had dubbed himself the "Great Leader."

The Korean and Cold Wars

In 1950, Kim Il-sung decided to try to reunify Korea under communist rule. He launched an invasion of South Korea, which turned into the three-year-long Korean War.

South Korea fought back against the North, supported by the United Nations and manned with troops from the United States. The conflict lasted from June 1950 to July 1953 and killed more than 3 million Koreans and U.N., and Chinese forces. A truce was signed at Panmunjom on July 27, 1953, and in it the two countries ended up back where they started, divided along the 38th parallel.

One upshot of the Korean War was the creation of the Demilitarized Zone at the 38th parallel. Electrified and constantly maintained by armed guards, it became a nearly impossible obstacle between the two countries. Hundreds of thousands of people fled the north prior to the DMZ, but afterward, the flow became a trickle of only four or five per year, and that restricted to elites who could either fly across the DMZ, or defect while out of the country.

During the Cold War, the countries continued to grow in different directions. By 1964, the Korean Workers' Party was in full control of the North, farmers were collectivized into cooperatives, and all commercial and industrial enterprises had been nationalized. South Korea remained committed to libertarian ideals and democracy, with a strong anti-communist attitude.

Widening Differences

In 1989, the Communist bloc abruptly collapsed, and the Soviet Union dissolved in 2001. North Korea lost its main economic and governmental support. The People's Republic of Korea replaced its communist underpinnings with a Juche socialist state, focused on the personality cult of the Kim family. From 1994 to 1998, a great famine struck North Korea. Despite food aid efforts by South Korea, the U.S., and China, North Korea suffered a death toll of at least 300,000, although estimates vary widely.

In 2002, the Gross Domestic Product per capita for the South was estimated to be 12 times that of the North; in 2009, a study found that North Korean preschoolers are smaller and weigh less than their South Korean counterparts. Energy shortfalls in the North led to the development of nuclear power, opening the door for the development of nuclear weaponry.

The language shared by Koreans has also changed, with each side borrowing terminology from English and Russian. A historic agreement by the two countries to maintain a dictionary of the national language was signed in 2004.

Long-Term Effects

And so, a rushed decision made by junior U.S. government officials in the heat and confusion of World War II's final days has resulted in the seemingly permanent creation of two warring neighbors. These neighbors have grown further and further apart, economically, socially, linguistically, and most of all ideologically.

More than 60 years and millions of lives later, the accidental division of North and South Korea continues to haunt the world, and the 38th parallel remains arguably the tensest border on Earth.