Literally the end of the world.
The war is fought sought in Africa. The US sells its ordinance to China then China uses its own against the US while the US runs out of supplies. The US government says nothing is wrong and that the war is fine.
China sells the US's bullets back to it at 400%. China then surrenders as the US is in economic turmoil.
Resident Cosplay Progressive
US nuke communist party headquarters and one or two major general, china surrender. China can do the same, but too much informal chain of command make us faster to recover and gain advantage.
If not, it's a total stalemate since no one can invade the other, except if new type of weapon or army of mechanized robots or one having aliens who come helping going independance day to the other.
To do good, people need to make an active response. And such inclinations aren't there. You have to want to care...think about the genocide in Darfur? Sure, it's evil...but no one cares when it doesn't make an impact on one's own country.
It would probably look like a ton of third world countries being sold arms and having puppet dictators setup. Then them fighting each other to the death. In a couple of these brush wars each side gets involved with directly. In those the other side ups the ante on arming and providing intelligence to the underdog.
That is how big economies fight other big economies now. Resetting humanity to the stone age benefits no one.
What does it look like? The USN/USMC duke it out against the PLAN/PLA in the SCS, but it ends with China launching all of their long range nukes, and the US launches a sizable portion (but not all, it needs enough to keep Russia at bay) of its in return. Both countries suffer massive damage (China more so), which leads to a world wide depression. Neither is in place to be a world power for many years.
Binkov's Battlegrounds on youtube does these various scenarios. Here's his US vs China