Korea: A Brief History Explains Everything
A good starting point for understanding the ongoing conflict between North and South Korea is the agreement between the United States and Japan in 1905, known as the Taft-Katsura Memorandum, which was signed as Japan was defeating Russia in the 1904-05 Russo-Japanese War. In that document the U.S. agreed to the Japanese colonization of Korea in trade for the American occupation of Hawaii (which the U.S. had annexed in 1898) and the Philippines (which the U.S. had acquired as booty in 1898 at the conclusion of the Spanish-American War). Koreans were not consulted about this arrangement.