Although they were unified for centuries under the Joseon Dynasty (1392 - 1910), and share the same language and essential culture, for the last six decades and more, North and South Korea have been divided along a fortified DMZ. How did that split come about? Why do North Korea and South Korea exist where once there stood a unified kingdom?
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, it became clear to the Allied Powers that they would have to take over administration of Japan's occupied territories, including Korea, until elections could be organized and local governments set up. The United States 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 US. 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 August 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 August 15, after the atomic bombing of Nagasaki, Emperor Hirohito announced Japan's surrender, ending World War II.
Just five days before Japan surrendered, US officials Dean Rusk and Charles Bonesteel were given the task of delineating the US 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 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 US Military Administration feared the leftist tendencies of many of the nominees. The trust administrators from the US and the USSR were supposed to arrange for nation-wide elections to reunify Korea in 1948, but neither side trusted the other. The US wanted the entire peninsula to be democratic and capitalist; the Soviets wanted it all to be communist.
In the end, the US essentially appointed the anti-communist leader Syngman Rhee to rule South Korea. The South declared itself a nation in May of 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 September 9, 1948. Kim began to squash 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."
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; it killed more than 3 million Koreans, but the two countries ended up back where they started, divided along the 38th parallel.
And so, a rushed decision made by junior US 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. More than sixty 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.