Why did the northern nomads invade the south in the early years of the Western Jin Dynasty?

First of all, in the early years of the Western Jin Dynasty, political chaos, especially the weakness of national strength after the Eight Kings Rebellion, gave northern nomads the opportunity to invade the south.

Another important reason is related to the climate.

Historically, the regional or ethnic regimes established by northern nomads in the Central Plains mostly appeared in the cold period. During the warm period, the high temperature and abundant precipitation are beneficial to agricultural production, and the whole social economy is relatively prosperous and politically stable. The nomadic people in the north have neither the necessity nor the possibility of moving south on a large scale.

However, in the cold climate, the precipitation is obviously reduced, and the temperate grassland will move south by about 200 kilometers. In order to survive, nomadic people in the north will inevitably move southward on a large scale with the southward movement of grasslands.

The dry and cold climate also destroyed the cultivation of the Han people, who are often thousands of miles away, and there was no harvest. Coupled with the extortion of the ruling class, the internal war of the Han nationality has started, greatly reducing the ability to resist the nomads from the south, making it a reality for northern nomads to enter the Central Plains.