What is the greatest feature of China's ancient national defense history?

Externally: Before the Song Dynasty, the main contradiction was the northern border incident caused by nomadic people, and there were no rivals in the other three directions: south, west and east.

During the Song, Yuan and Ming Dynasties, the northern border troubles still existed, but the emerging Hongyi cannon was the nemesis of nomadic cavalry, and the threat of the northern border troubles declined. However, harassment and threats from the sea have greatly increased.

The main internal contradiction is the relationship between the central army and the local vassal army to split the national defense relations between countries, and the problem of government forces preventing and suppressing peasant uprisings.