In ancient times, the greatest threat to the Central Plains Dynasty came from the nomadic peoples of the north. In the period of great power, successive dynasties would choose to use troops against the northern nomads to eliminate the threat as much as possible, such as Qin Shi Huang, Han Wudi, Tang Taizong, Ming Taizu and so on.
The Qing emperor was more ruthless towards the nomads. The three emperors of Kangqianyong preferred to negotiate peace with Tsarist Russia and suffer losses in the demarcation of the border, and also concentrated their resources to eliminate the Dzungars. During the Qianlong period, the Dzungar clan had already declined, and it could not pose a threat to the Qing Dynasty in the short term, so Qianlong still sent troops to exterminate the Dzungar clan. The Qing Dynasty also demarcated the mongol tribes and vigorously promoted Tibetan Buddhism in Mongolia to limit the number of Mongols to have children, in order to eliminate the threat of nomads to the Central Plains Dynasty.
Why did the northern nomads pose such a great threat to the Central Plains Dynasty?
Let me give you two examples.
From the beginning of the Qin Dynasty, Chinese established rule in northern Vietnam, and after Emperor Wu of the Han Dynasty annexed the State of South Vietnam, he established a solid rule in Vietnam. Later, why did Vietnam become independent from China?
Because Vietnam is too far from the center of Chinese rule, and The Emperor of Shangao is far away. Whenever there was chaos in the Central Plains, local Vietnamese magistrates and magnates wanted to close the barrier to the Central Plains and divide one side. During the Three Kingdoms period, Jiaotong Taishou Shi Xie ruled the area for 40 years, and after Shi Xie's death, his son wanted to be independent. Fortunately, The Wu lord Sun Quan had already taken precautions and sent troops to raid to regain control of Jiaotong. But Vietnam's independence is only a matter of time.
By the Time of the Five Dynasties, Vietnam was still independent from China. No way, the geographical location is too far away, at the very edge of the central plains dynasty rule, one day it will be independent.
If the Central Plains Dynasty is a strong crossbow with a range of 1000 meters, Vietnam happens to be in the position of 1000 meters, and the crossbow arrow is already the end of the strong crossbow in Vietnam, and it is difficult to cause large damage. Vietnam thus became independent.
The second example is that Cai Yi launched a patriotic war, forcing Yuan Shikai to abolish the imperial system.
In December 1915, Yuan Shikai ascended the throne as emperor. Cai Yi, Tang Jiyao, and others launched the "Patriotic War" in Yunnan and rebelled against Yuan. At that time, Yuan Shikai could command hundreds of thousands of troops, and the Dian army only had 20,000 people, of which more than 5,000 were still urgent recruits, and the strength of the two sides was very different, but the Beiyang Army was unable to defeat the Dian army.
Not because the Beiyang Army is not strong, but mainly because of geography. The main force of the Beiyang Army was mainly in the north, and there were few stations in the southern provinces. Yunnan is geographically remote, the southwest region is densely forested with mountains, and it is on the edge of the control of the Beiyang government, and it is impossible for Yuan Shikai to eliminate the defenders militarily in a short period of time. In the end, Yuan Shikai could not withstand the pressure and announced the abolition of the imperial system, and the patriotic war was successful.
For the Central Plains Dynasty, the nomads were similar to the above examples. Vietnam and Yunnan are at least within the scope of their rule, but they are on the margins and not easily managed. The nomadic people were outside the jurisdiction of the Central Plains Dynasty, and many times they were not ruled by the Central Plains Emperor at all.
From the great wall line to Lake Baikal, from the Daxing'an Mountains to the steppes of Eastern Europe, it is the scope of activity of nomadic peoples. Sometimes far away, sometimes close, the Central Plains Dynasty, even if heavily defended in the north, was still unable to eliminate the threat posed by the nomads.
What is even more frightening is that China's capital is generally in the north, and although the "Son of Heaven guarding the gate" can effectively prevent the nomads from going south, it also makes the capital vulnerable to the threat of nomads. Once the nomadic cavalry crossed the Great Wall, they could reach Beijing or Chang'an in a matter of hours.
It was difficult for the Central Plains Dynasty to crack down on the nomads. Sending too few troops cannot be fought, and the logistics of sending more than 100,000 troops is a big problem. To transport grain and grass through deserts and grasslands, more than 100,000 horses and hundreds of thousands of grain workers are needed. There is no return from the blood transfusion, there is not much loot to win the war, and it is impossible to establish a long-term effective rule, wasting the lives of warriors and countless money and food in vain. Whether you win or lose, it is a huge loss economically.
Therefore, in the case of economic and technological backwardness in ancient times, it was difficult for the Central Plains Dynasty to completely solve the threat posed by nomadic peoples, especially the Han Dynasty.
You can also say your own opinion.