Relations between The Democratic People's Republic of Korea (North Korea) and The Republic of Korea (South Korea) haven't always been as strained and controversial as they are today. In fact, the division of the two countries is a relatively recent phenomenon. Before World War II, Korea was united in language, culture, and politics for over a millennium.
Here is an overview of some of the major historical events in the Korean peninsula up to its political division at the 38th parallel:
668 AD: The Silla unite the three kingdoms of the Korean peninsula — Silla, Paekche, and Koguryo — establishing the Silla Dynasty and beginning a decade of fighting to expel the forces of China's T'ang Dynasty.
936: General Wang Kon overthrows the Silla and establishes the Koryo Dynasty. The English word Korea comes from Koryo.
1392: General Yi Song-gye, a Confucian scholar, overthrows the Koryo dynasty and founds the Yi Dynasty. The kingdom is renamed Choson.
1443: Hangul, the Korean phonetic alphabet, is invented.
1592: Japan invades Korea. With help from China, Korea repels the Japanese forces.
1627: Manchuria attacks northwestern Korea in preparation for a larger attack on China.
The battles in 1592 and 1627 result in the loss of many cultural artifacts and assets and the weakening of the centralized government.
1876: Following a treaty with Japan, Korea's mostly isolationist policies shift, and the peninsula is opened up to the West.
1894: The Donghak Peasant Revolution, a series of anti-feudal attacks on government officers. The Korean government asks for aid from China. The Qing Dynasty of China sends 2,700 soldiers to help without alerting the Japanese, in violation of the Treaty of Tientsin.
This begins the Sino-Japanese War for control of Korea between its two co-protectorates, China and Japan.
April 17, 1895: The Treaty of Shimonoseki ends the Sino-Japanese war; Korea gains independence.
1904-05: The Russo-Japanese War. Japan defeats Russia in a battle for influence in Korea and Manchuria.
1910: Japan formally annexes Korea, bringing the Yi Dynasty to an end. Japan makes efforts to supplant Korean language and culture.
1945: World War II ends, and Korea is divided at the 38th parallel. Administration of the two halves is divided temporarily, with the USSR administering the northern half and the USA administering the south.
1948: Two separate nations — North Korea and South Korea — are formally established.