In fact, it is mainly because of the age of Xuanzong in Tang Dynasty. As he grows older, his energy and thoughts will change. In fact, I always thought he was not a first-class emperor. The emergence of the flourishing age of Kaiyuan was mainly due to the fact that many outstanding emperors had paved the way before Xuanzong of the Tang Dynasty. For example, Emperor Wendi, Yang Di, Emperor Taizong and Wu Zetian all paved the way for this. Although there were regime changes and court coups in the middle, China generally developed upward. In other words, if there is no big change, as long as there is a promising monarch, the emergence of the prosperous era of Kaiyuan is a historical necessity.