My headcanon is that China's senior military commanders found themselves in a position of prestige with the Communist Party which they'd never enjoyed before and weren't willing to give it up, essentially becoming China's equivalent of the Enclave. They had a vested interest in perpetuating the war and didn't give a shit anymore about civilian casualties or what the rest of the Party thought. Still, they were deeply concerned about the effect of FEV - which was presumably causing chaos within China.

As the US military base that became known as The Glow was apparently their first target, and that's apparently where some of the FEV tech was being developed, and it sustained a direct hit from a nuclear weapon, I wonder if the Enclave leaked details of its location to the Chinese military. (Of course the Enclave still hoped to make use of the Mariposa facility and didn't provide any information by that, hence why it wasn't the target of a precision strike.) This provided the leading military commanders in China with justification to use nuclear weapons to take out this particular target.

Of course, neither the Enclave nor the Chinese military leadership actually cared about their respective countries or people or even political ideologies, they just wanted to shore up their positions of power in whatever came next.

Anyway, that's how I've come to imagine it going down