Korea wasn't always divided into North and South like it is now. While today the differences between the two seem as distinct as night and day, just 75 years ago they were essentially the same nation. Why then, is Korea divided into two countries?
The history of North and South Korea is complicated. The country was split in two after WWII, but the story really goes back to a time long before that. The history of the Korean Peninsula is a tale of a small kingdom being pushed around by much larger powers again and again. The split between the North and the South is just the latest manifestation of that phenomenon.
From 1392-1910 Korea Was A Relatively Unified Kingdom
For nearly 5 centuries, the Korean Peninsula was ruled by the Joseon Dynasty with varying degrees of authority. The Joseon Dynasty arose after many years of decline on the part of the 400-year-old Goryeo Dynasty and a few years of nominal Mongol occupation. Long story short, an ambitious general named Yi Seong-gye staged a coup, and - after a few years of puppet rule - installed himself as the new King. Going by the moniker King Taejo, he moved the capital to Seoul (then Hanseong).
The Joseon Dynasty's power wasn't always absolute over their 500-year reign, however. In 1401, it became a tributary of Ming China. In 1592 and 1597, Japan invaded, but they didn't stay around long. In 1637, the Joseon lost a war with the newly established Qing Dynasty in China and was forced again to become their tributary.
By the end of the 19th century, the Joseon Dynasty was in decline. Both the Donghak peasant rebellion and increasingly untenable isolationist policies left the central government weak. At this point, both Chinese and Japanese forces increased their presence in Korea, leading to the First Sino-Japanese War (fought mainly on Korean soil). After Qing China's defeat at the hands of Japan in 1895, Korea then became a tributary of Japan.
Korea Was A Colony Of Japan From 1910 To 1945
Nominal Japanese rule over Korea actually began after the First Sino-Japanese War in 1895, when Korea essentially changed from a tributary of Qing China to a tributary of Japan. From the start, the Japanese systematically imposed rule on the declining Joseon Dynasty. This culminated in the Japan-Korea Annexation Treaty of 1910, in which the Emperor of Korea formally ceded all authority to the Emperor of Japan... though he refused to sign it himself and instead sent an ambassador to Japan on his behalf.
For the first 10 years, Japan imposed harsh military rule on the new colony. In 1919, a nationwide protest demanding independence was met with overwhelming force. Different estimates place the number of protesters killed at between 553 and 7,509. After the incident, the Japanese revised and somewhat relaxed their policy of rule. Korea was rapidly modernized and industrialized in order to provide Japan with goods and resources to fuel their expansionism.
During the latter part of the 1930s, as Japan mobilized for war, conditions again became harsh. The Japanese decided to try to assimilate the Koreans, teaching school in Japanese. Koreans were made to adopt Japanese names, and after 1938, schoolchildren were actually forbidden to speak Korean. Efforts were made to eradicate native Korean culture, and Shinto was introduced, but it didn't take.
The Russians And Americans Divided Korea Into Two Zones
After Japan's defeat in WWII, the question remained as to what to do with Japan's former territories. The US had already taken many of Japan's island holdings, and, in the last days of the war, the Soviet Union had overrun much of Japan's territory in Manchurian China.
Worried that the Soviet Union would soon dominate the whole of Korea as well, the US proposed splitting it into two zones. Days before the war closed, the two nations agreed to split control of postwar Korea along the 38th parallel, putting Seoul in the American half. The idea, of course, was to later unify it under a newly established government.
In 1948, Two Governments Were Formed In The Two Zones
Initially, idea was to have the whole of Korea unified under one government when it was split in the wake of WWII, except the occupying US and Soviet forces couldn't agree if said government would be capitalist and democratic or communist. Notably, Korea had never been either before, only having experience as a caste-based monarchy and a subservient colony, respectively.
To solve the impasse, the US basically set up anti-communist Syngman Rhee as the President of the Republic of Korea with United Nations sanctioned elections in August. In September, the Soviet Union set up former Soviet Red Army Major Kim Il-sung as the Premier of the Democratic People’s Republic of Korea (in the Stalinist, "cult-of-personality" model). Each of the two governments claimed sovereignty over the whole of the Korean Peninsula.
The Chinese Communist Revolution Acted As A Precursor To The Korean War
Meanwhile, in neighboring China, things didn't all go smoothly after the end of WWII. The Chinese Communist Party (CCP) and the Nationalist Party, or Kuomintang, had been at odds since the 1920s, and without the common enemy of Japan, tensions between the two rapidly worsened and turned into a full-scale civil war by 1946. Logically, the Soviet Union backed the CCP, and the US backed the Nationalists, both sending resources and assistance.
Due largely to many previous years of mismanagement, corruption, and loss of popular support, the Nationalists lost the war to the now surging CCP. In 1949, Mao Zedong declared the establishment of the People's Republic of China, marking the Chinese Communist Revolution. Meanwhile, the Nationalists fled to Taiwan, under the protection of the US. This would have direct ramifications on the two Koreas.
In 1950, Communist North Korea Invaded The South
Seeing the success of Mao in China, Kim Il-sung decided to embark on a Korean reunification effort. In June, thousands of troops, armed with Russian tanks, guns, and planes, flooded over the 38th parallel, marking the beginning of the Korean War.
The newly formed United Nations Security Council then branded the North as the aggressor. The British Far East Fleet was drawn into service, and the US began sending in troops. The North's advance was stopped at the port city of Pusan. As reinforcements arrived, UN forces pushed northward. Under the command of MacArthur, UN forces were granted permission to cross the 38th parallel and attempt to reunify Korea as the Republic of Korea. By the end of October, they controlled most of the Peninsula.
Maoist China Intervened On The Side of North Korea
Just as the Korean War appeared to be over, China sent its armies into Korea in November 1950 to fight the UN forces. Even after initial attacks, the UN forces led by MacArthur continued to discount the size of the Chinese force, which was 300,000 strong. Overwhelmed by the human-wave tactics used by the Chinese Communist forces, UN forces were turned back in disarray.
By the beginning of the new year, they were defending a line south of Seoul. The line held, however, and the UN forces were again able to push north to the 38th parallel.
Korea Would've Been Reunited By Nuclear Fallout If MacArthur Had His Way
Given China's intervention into the war, MacArthur came up with a surefire strategy to win under the new circumstances: 30 to 50 atomic bomb drops. They would be targeted at completely destroying both China's and North Korea's air force. This done, he would then spread a defensive perimeter of radioactive cobalt along the Chinese-Korean border, preventing any further land invasion. Their supply lines cut, the North Korea army would be quickly starved into submission.
Truman and the Joint Chiefs of Staff were not on board. Besides fears that Russia would enter the war, starting WWIII, they really didn't want to carve a swath of radioactive fallout across east Asia. Instead, they opted to fire MacArthur in April 1951.