668 -- The Goguryeo Kingdom, which was established in 37 B.C., is conquered by troops from a coalition of Korea's Silla Kingdom and China's Tang Dynasty. Considered the forerunner of modern-day Korea, the Goguryeo Kingdom stretched from areas of what is now North Korea to Manchuria in northeast China. Royal tombs, murals, castles and monuments from the Goguryeo period were designated as UNESCO World Heritage sites in July 2004.