Sword can only be strong for a while, such as Xiongnu in Attila, Mongolia in Genghis Khan, former Qin in Fu Jian, Macedonia in Alexander the Great, and powerful Roman legions. So many powerful legions, they either annexed advanced civilization and eventually assimilated it, or were defeated and disintegrated in a protracted war. The most obvious is the Mongolian Empire. Although it lasted for a long time, it annexed too many complementary advanced civilizations in its own territory, and finally it became a system, leading to division. In the process of assimilation, backward civilizations were corroded until empires fell apart one after another.