Li Mu(李牧) was arguably the last great general to have come from the Kingdom of Zhao during the
Warring States period. After the death of Zhao Sheand the exile of Lian Po, two of Zhao's great generals; he assumed command of Zhao's overall security situation, which by this time was becoming extremely critical with the rapidly expanding Qin to the west and the Xiongnuto the north. In the beginning Li Mu was more concerned with the incessant Hunnish raids on the Zhao border, and led several campaigns against them; later as the threat from Qin increased with the ascension of Qin Shi Huang, he turned his focus more to the western parts of Zhao.
The Zhao Kingdom, by this time, was merely a shadow of its former self. Its army was basically annihilated during the
Battle of Changping, a blow which it never recovered from, unlike its victorious opponent Qin, it was diplomatically isolated from the Kingdoms of Wei, Yan and Han too weak to offer any king of support and Qi and Chu were more willing to see the kingdom extinguished than face the powerful Qin together. Most of its core territories had fallen to Qin earlier in the aftermath of Changping, with only several cities of note.
Li Mu, however, with such limited resources, could still hold out against the enemy forces; though, admittedly, he wasn't facing anyone who had anywhere near the talent of
Bai Qi( Meng Tianat this point was still not yet a general), however, judging from the situation, it was still no mean feat. So while Qin could romp around in Wei and Han, in the Zhao Kingdom, they had a much harder time pillaging.
Getting rid of Li Mu became a necessity if Qin ever hoped to conquer Zhao and unify
China. So Qin sent spies to the Zhao court, bribing key courtiers in order to persuade the King of Zhao that Li Mu was planning to revolt and should be gotten rid of immediately. The plan succeeded brilliantly - Li Mu was arrested and soon executed on the king's orders.
With Li Mu's death the annexation of Zhao became inevitable. In about 10 years King Dai would fall and the State of Zhao would fade into history.
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