As the title suggested, the historical description of the battle of ChangPing says that both Qin and Zhao sent an army in the size of about 500000 men. And after Zhao lost the battle, Qin buried 400000 Zhao soldiers alive.
I find such figures very difficult to believe. For comparison, the army of Alexander the Great was estimated to consist of ~50000 soldiers. I've read some sources that suggest the population of Qin and Zhao were about a few millions, how is it possible that they could support an army of this size with this population? How could they produce enough food and equipments to keep it supplied? Even if they had surrendered, how could Qin restraint and bury 400000 men alive? That would take a lot of time and work and surely once they had realised what Qin has planned for them they would resist and fight back?
After a brief search on the internet it seems to me that such figures were not heavily challenged. There're two similar questions asked on this sub but one of the answers was deleted, and the other one doesn't include a lot of credible sources (the author only included a link to baidu which imho is not a credible source).
Many battles in the Warring States and Han period seem to have involved armies in sizes that are hard to believe. My question is, how historically accurate is the description of the battle of ChangPing? Is it exaggerated, if so what is the modern estimations? What is the academic consensus on this?
I'll preface this by saying I am by no means an expert on Warring States China, nor am I an archaeologist. What I did do is to look at the Chinese language sources on this. And Chinese scholars have long cast doubt on the 400,000 and 500,000 figures given in the Shiji, but there has been no consensus on the exact figures. Historians estimates that Zhao had a population of 4-7 million, so 400,000 would have meant 10% of their population mobilized (if we go by the low figure). Very unlikely.
I argued several years back that states like Qin and Han had the potential to mobilize upwards of one million men for combat. This was due to the practice of universal conscription and a highly centralized bureaucracy capable of massive mobilization and logistical support. The Qin conquest of Sichuan and construction of various irrigation canals also opened up agricultural land which made it able to support mass mobilization. But potential doesn't mean it's practical and logistically feasible, so the Qin never did mobilize that many men. But mobilization of 100,000 or even 200,000 was entirely possible. It's also important to note that these massive armies rarely fought as one single force. Instead, they were divided into many smaller contingents fighting on different fronts and mobilized at different points in time. Most of the times wars were meant to be short and decisive and were fought near borders which made logistical support easier, and so states could afford to mobilize large armies. To give an example from the Han, the famous Battle of Mobei actually consisted of two cavalry divisions of 50,000 each and they were supported by 200,000 infantry acting as logistical and rearguard support and 40,000 pack horses.
If we apply that principle to Changping, then it makes more sense. Qin and Zhao probably did not start with armies that massive, but continually reinforced their armies with fresh troops as time went on. Also it's important to note that the figures probably did not include just soldiers. Peasants conscripted for logistical support, porters, laborers were probably all included. Changping was also an exception because it lasted three years whereas normal battles on that scale would have lasted a couple months at most. In fact, Lian Po's entire strategy was to wear down the Qin forces and exhaust them, since Changping was much closer to Zhao borders, which made it easier for Zhao to resupply but difficult for Qin. Qin, being the more economically stronger of the two states, was able to absorb the demand that the campaign brought.
Finally, to address the "burial" of Zhao soldiers. Sima Qian uses the character 坑, which has the meaning of "to bury" in Classical Chinese to describe what Bai Qi did to the Zhao soldiers. Later historians took it as meaning buried alive, but archeological evidence from the Changping battle site suggests otherwise. For example, many bones were found with cut marks, and many bodies were found buried with arrowheads. This suggests that the Zhao soldiers were killed first and then buried.
TL;DR: 400,000 and 500,000 figures are possible but highly unrealistic, and no one knows the exact figures. Historians and archeologists do agree that it's a massive battle. I'd estimate that Qin and Zhao probably put half that number into combat. Zhao soldiers were also killed first and then buried.