From Paekche, Buddhist monks were sent to Japan in the 6th century. This is the point where Buddhism was introduced to the island nation. The Buddhist monk, Kwalluk, (Kanroku in Japanese), crossed the East Sea in 602 AD. He brought with him a large number of Buddhist sutras, historical books, works on astronomy, geography, and the occult arts, including the science of Ki. He was instrumental in the founding the Sanron school of Buddhism in Japan.
By the end of the sixth century, Buddhism reached its maturity in all three Kingdoms on the Korean Peninsula. Korean monks were commonly sent to China and India, and missionaries frequently traveling to Japan. Buddhism completely replaced the primal shamanistic religions indigenous to the Korean Peninsula.
Though The Three Kingdoms had become essentially Buddhist in their religious beliefs, there existed ever-increasing differences in their political ideologies. Warfare between the kingdoms, though always present, moved onto new expansionist ideologies.
Koguryo continued its expansion through the Korean peninsula, using its elite military class, the "kyon-dang." Its expansion reached a peak, in the fifth century AD, when it had gained control of half of the Korean peninsula and much of Manchuria. With the start of invasions by Koguryo and the hostile Han (Chinese) tribes from the north, and Paekche's growth from the west, Silla was compelled to defend itself.