From the 1st century A.D., three ancient kingdoms emerged with Koguryo at the head, and Paekche and Shilla soon following. Koguryo developed into a powerful nation and acted as a bulwark against the aggressive northern nations and China. During this time, the kingdoms of Paekche and Shilla developed rapidly. This increased friction between the Three Kingdoms. These Three Kingdoms were unified by the last of the developed kingdoms, Shilla, thereby marking the first national unity. On the other hand, in Manchuria, the old cradle of Koguryo, Parhae was firmly established and undergoing distinct development. The Three Kingdoms developed as aristocrat-centered societies, and the aristocrats became the leaders of the culture. The Three Kingdoms accepted Buddhism, which greatly advanced and expanded the scope of their culture. In this way, the Three Kingdoms developed a colorful and refined aristocrat-centered Buddhist culture which was eventually introduced to Japan and greatly influenced the development of its ancient culture.