During WWII the Japanese insisted the Koreans fight on their side, but few agreed to this and many joined the Chinese army to liberate themselves from Japanese rule. Due to their geographic location, the Korean peninsula was the victim of Chinese-Japanese battles and by war's end the peninsula was in poor condition. To put a bigger strain on the economy, many of those Koreans who fled under Japanese rule returned to the peninsula.
At the conclusion of WWII, the peninsula was divided between the United States (in the south) and the Soviet Union (in the north) in the form of administration zones, but with the idea that the two sides would unite. The United Nations (UN) led a peninsula-wide popular election to determine future political governance, but the north refused to participate. Once results were tallied, the south declared independence as the "Republic of Korea" and the north countered by claiming independence as the "People's Democratic Republic of Korea;" both side claimed jurisdiction over the entire peninsula.
This political tension rose in the late 1940s until the Korean War broke out in 1950. After a surprise attack and quick advance into the south, the north fell back as the UN and US landed troops on the peninsula. To respond, China and the Soviet Union (although unofficially) entered the war and in 1953 the war ceased in a stalemate with a new border almost exactly where the original border had been.
After the Korean War the government in South Korea became more autocratic and repressive as the economy declined and multiple leaders were overthrown only to be replaced by other autocratic rulers.
This stagnant rule ended in the late 1970s with the assassination of South Korea's President Park Chung-hee. The new government came in and greatly improved the economy, while opening relations with the north to discuss a political union. These talks went nowhere and soon the economy was again failing so in the late 1980s a new government was brought in.
Since the 1980s the state of affairs in South Korea has slowly improved as their economy is opening up and expanding while political freedoms are being extended and international communications have improved. However, the situation in relation to North Korea is still unresolved and the people in the south today debate what the best direction for their future is. Many people maintain unity, while many young people view a joint state would be little more than an economic burden, crippling the future of the nation.