As a result of the high frequency of warfare experienced in Northeast Asia, and in particular the destructive nature of the large number of invasions suffered by the Korean Peninsula, the majority of any documentation of Korean indigenous martial techniques and combat styles has been lost. Much of modern-day scholarship related to the classical or pre-modern Korean martial arts, in particular those that predate the Mongol invasions of Korea (during which Korean palaces and libraries that would have contained many documents germane to the study of martial arts and combat self-defense were destroyed), relies on the study of the "Samguk Sagi" compiled by the scholar Kim Bu-sik and the "Samguk Yusa" compiled by the Buddhist monk Iryeon. Both works suggest that militant attitudes between and among the three major nations of the Korean Three Kingdoms Period (37 BCE – 660 AD) resulted in each nation developing an institution for training its warriors in martial arts and military sciences.