Leaving the Great Tang Empire nothing but ruins, a blank land... This seems to be a good idea. By then, the homeless civilians can only seek help from the Tang Army, which will undoubtedly halt the Tang Army's advance.
Moreover, a city with nearly a million people, without water and electricity, will immediately collapse. The Tang Army occupying such a city would inevitably have to find ways to restore order and appease the populace... They would not have the strength to hunt down the retreating Qin Army.
After all, the Qin Army would not destroy the cities under its control, they only carry out such crazed orders before retreating, so for the Qin Army, this is almost cost-free.
Emperor Qin Ying Duo looked at Liu Zhian, whose words were indeed cruel, but to him, they did seem somewhat feasible.
The effect, of course, goes without saying. As long as the Qin Army goes all out during the retreat, the Tang Army will surely be left with a mess.
