In its heyday, the Huns unified Saibei, and each tribe was a relative. This unification process is naturally long. Therefore, it's not that we think the Xiongnu are fighting more and more, but that the Xiongnu tribes are gradually unified and growing without interference, and once they recover their vitality, they will attack the Central Plains again.
Xiongnu was originally a nomadic people in the Mongolian Plateau. In the heyday of Qin Dynasty, Xiongnu was driven out of Hexi Corridor by Meng Tian, the general of Qin Dynasty. However, at the time of the demise of the Qin Dynasty, the people of the Central Plains were in urgent need of recovery because of years of war. The Huns took this opportunity to make a comeback and wanted to occupy the whole Central Plains. They fought a tug-of-war with the newly established Western Han regime, and the intermittent struggle continued until Emperor Wu of the Han Dynasty.
When Sima Qian studied the history of the Huns, he pointed out the Huns' own statement: "They are also descendants of Xia Houshi." To some extent, it shows that Huns were originally descendants of Xia Dynasty, but there is no exact statement about the specific truth.
Central plains people have always looked down on the Huns, but they are afraid of them. Because I can't afford a protracted war, I always rush to a certain border, which is equivalent to peace for a while. In fact, there has never been a real extinction. As for the decline of Xiongnu, on the one hand, it is the tenacious resistance in the Central Plains, on the other hand, it is because the Baiyi people are not easy to rule and civil strife is frequent, which leads to the failure of a certain notification group for a long time.
Nomads have only nationalities, no cities and villages, no poetry and wine, no chess and calligraphy, and no property. It's just the primitive consanguinity and unity that drives cattle and sheep to the east today and appear in the west tomorrow. The war between farming people and nomadic people will always be asymmetric, utilitarian and impossible to achieve. So we have been fighting the Huns, and finally the Huns retreated from the whole division, entered the fertile soil of Europe, destroyed western Rome, and established the Hungarian country!