This period constituted roughly 20 regimes. However, the Tang dynasty formed a large part of it and played an important role not only in Chinese history as the second imperial age after the Han dynasty, but also in Asia and in world history for its political authority and cultural influence over surrounding countries. China fell into disunity after the collapse of the Han dynasty. The nomads migrated into northern China and mixed with the Han people. The Sui, based on an army made up of mixed ethnic origins, conquered southern China and ended the division. Then, the Tang succeeded the centralized policy of the Sui and successfully built a great empire of wealth and conquest. However, the Tang's decline also resulted in political disintegration. Although the situation deteriorated to the degree that many regimes co‐existed during the Five dynasties, it did not last long until the reunification of the Song dynasty.