After hundreds of years of annexation wars in the Spring and Autumn Period, in the Warring States Period, there were only seven countries left in the Spring and Autumn Hundred Kingdoms, and these seven countries can be called superpowers in the region, known in history as the "Seven Heroes of the Warring States".
Among them, the Zhao, Yan and other states in the Warring States period were all threatened militarily by the northern nomads to varying degrees, but the countries were able to repel the invaders and also sent troops to win major victories, and in the early Han Dynasty, when the Great Unification was unified, the Han Court suffered from the military aggression of the Xiongnu, and even did not hesitate to humiliate the land and relatives.
First of all, during the Spring and Autumn Period and the Warring States Period, the Xiongnu were not a unified nomadic tribe, there was no unified leadership and command, and each tribe was scattered and independent, so although there were many tribes within them, their individual strength was very weak. A tribe usually has only a few thousand people, and a larger tribe may have tens of thousands of people, and although the Zhao state is not large, it is also a large country with a population of one million, and its strength far exceeds that of the Xiongnu tribes.
However, with the continuous annexation wars of nomadic tribes, the Huns also began to move towards unity, and in 209 AD, that is, the first year of the Qin Dynasty II, Mao Dundan unified the northern steppes and officially established the Xiongnu Empire. Through decades of external expansion, in the early years of the Han Dynasty, the Xiongnu Empire reached the peak of sitting on the ground for thousands of miles and supporting 400,000 troops, and at the beginning of the Han Dynasty in the same period, it experienced the peasant war at the end of the Qin Dynasty and the dispute between Chu and Han, and the Han Empire had no advantages in economy and military, so it could only carry out humiliating diplomacy.
Secondly, the military strength of the Zhao state should not be underestimated, and it was not even weaker than that of the Qin state in the same period, not to mention the Unsized Xiongnu.
Since the military reform of King Hufu of Zhao Wuling, the military strength of the Zhao State can be called the highest among the Seven Kingdoms, and the number of famous generals (such as Li Mu and Lian Po) is huge, and the quality is also good, and equipped with mobile and flexible cavalry and chariot phalanx, it can be said that there is no opponent. If in the later Battle of Changping, the Qin army was not advantaged by a million troops, and the commander of the Zhao army, Zhao Kuo, was incompetent, it would be difficult for the Qin state to swallow the Zhao state in one bite.
Although the han army in the early Han Dynasty was not weak in battle, it was consumed by civil unrest. After the end of the Chu-Han War, although the Han Empire was established, but internal rebellions continued, Yingbu, Peng Yue and other kings with different surnames rebelled one after another, although they were later pacified, but also caused the Han Dynasty to lose troops, capable generals either rebelled or were killed out of suspicion, and the Han Dynasty fell into an embarrassing situation where there were no marshals to use.
In addition, the borders of the Zhao state were short and there were geographical barriers, so it was difficult for the Xiongnu to win.
The northern part of the territory of the Zhao State not only had the natural dangers of the Yellow River, but also the barrier of the Yinshan Mountains, and the Zhao State also built the Great Wall to resist external troubles, so it was difficult for the Xiongnu to break through the Zhao Territory.
Moreover, the border defense line of the Zhao State is far less wide than that of the Han Dynasty, during the Warring States period, it was the three kingdoms of Qin, Zhao and Yan to jointly defend the northern defense line, so it was enough for each country to take care of its own military defense line, and the Han Dynasty wanted to defend the entire northern defense line from Shaanxi and Gansu to Liaodong, so it was much more difficult.