Invincible army? Not so much, but probably not the curbstomp everyone assumes it'll be. Zombie armies and even powerful intelligent undead get stopped all the time in D&D, usually by a plucky group of 4-8 adventurers.
EDIT:
I'd imagine that if they revive a bit worse each time, then there is a finite number of times they can revive before the army is just bones that are literally falling apart. It might be a large number, but still finite. Plus the city DOES have multiple individuals who should be able to Turn Undead.
*Pinches nose* But here's the thing... we keep hearing about how this is going to be an absolutely massive fight, how this army is getting freaking hyped to hell and back with how strong they are, that the
US Military on
American soil alongside several ships who are from a Kardashev Type II civ, along side multiple very powerful dragons alongside other beings will be having a lot of trouble with said army continually able to resurrect. Whenever someone makes a point regarding a modern military, we then get "Well, yeah, but these guys can ______ and thus easily counter them."
It becomes hard to see
how one is going to be able to bring it to a good conclusion without it being either a massive let down or, as mentioned, a Deus ex Machina being involved somehow.
Granted, some bits... just make me scratch my head. Like how they
know there's an army marching in their direction, while we see characters continuing to go to school and the like. There's no evacuations for civilians or anything, not even at the hospital. And no real explanation for
why the US is not getting as many civilians as they can out of the way. We're not seeing a Watsonian explanation for that nor for why they have not begun to hit them already as far out as they are.
Doyalist, it makes sense as it is obvious to have a big clash, but still.
As an aside... bringing in "Oh, zombie armies" with regards to the US military and it having them easily match them makes me cringe as it brings up
very bad memories of one particular fictional battle...