Since the pre-Qin era, the Turks had been a formidable threat to the Central Plains.
After all, they were nomads, and nomadic peoples had three most notable traits: they relied primarily on herding and did not engage in farming; they moved with the grass and had no fixed homes; and they excelled at riding and archery, so nearly everyone was an elite cavalryman, and every household had plenty of warhorses!
That last trait—"excelling at riding and archery"—was especially significant. Once tens of thousands or even hundreds of thousands of nomads were rallied together, they became a terrifying force!
On open plains, in a head-on confrontation, armies of the Central Plains' agrarian civilization, composed mainly of infantry, were almost always no match for those fierce nomadic armies that could come and go like the wind while fighting on horseback.
Otherwise, during the Warring States period, Qin, Zhao, and Yan would not have built one stretch of the Great Wall after another along the ridges of towering mountains to defend against the northern Xiongnu. And after unifying the realm, Qin Shi Huang would not have continued the work and joined the original walls of Qin, Zhao, and Yan into one.
Of course, by the chaotic final years of the Sui dynasty, the northern nomads were no longer called the Xiongnu. They were called the "Turks." But the Turks and the Xiongnu shared the same roots; the Turks were simply a separate branch of the Xiongnu. The concept of "Turks" had only gradually emerged after the Turkic Empire was formed. It was rather like the succession of dynasties in the Central Plains, wasn't it?
But no matter how the dynasties changed, and whether they were called the Xiongnu or the Turks, in that era, nomadic peoples were always a pressing threat to agrarian