Also, can you recommend me some reading material for the Korean War, Korean history or even Asian history in General? Countries I'm specifically looking to find out more about are China, Japan, Korea, Mongolia and Tibet, but any reading material for Asian history will be greatly appreciated.
Korea was separated into 3 countries before the 7th century. http://en.wikipedia.org/wiki/Three_Kingdoms_of_Korea Goguryeo is modern day NK plus a bit of China and north parts of SK; Baekje and Silla is most part of SK. However unlike the 20th century version, the southern kingdom Silla is allied to China (Tang) whilel Goguryeo was against Tang. In 668, a Tang-Silla alliance took down Goguryeo. (Baekje is conquered in 660)
During early 10th century, due to Tang being weakened and eventually collapsing, the "descendents" of Goguryeo and Baekje "revived" their country. http://en.wikipedia.org/wiki/Later_Three_Kingdoms Eventually, the "later Goguryeo", renamed "Goryeo" then, unified the peninsula and the unification lasted until Japanese occupation.
Edit: recently, China has a dispute with BOTH North and South Korea about the history identity of Goguryeo. The Koreans of course say they were a Korean people and spoke ancient Korean, while some Chinese scholars suggest this country may be Tungusic (i.e. closer to Manchu) rather than Korean, it's the "later Goguryeo" that just took the name.