Or one side or the other kills each other off. See the Finnish Civil War for example.
I think it's unlikely that any situation such as is envisioned in North America in the XXI Century is likely to conclude after just four months, as Finland's did, though we can hope for such a short duration.
But note that during those four months in 1918, the Finns managed to kill off about 1% of their national population- it took Americans four years of the bloodletting of the U.S. Civil War/ War for Northern Dominance to manage a 2% natiuonal Butcher's Bill.
Too, only about 25% of the dead in the Finnish 1918 example were combat casualties- the rest were either prison camp deaths under freezing conditions, or field executions of enemy captives, sometimes by shooting, sometimes with bayonets, both to save ammo and to give the young rookies a little first-hand experience.
Let's see: the current U.S. population is around 330 million, give or take 10 or 20 million Mexican refugees. And a 1% or 2% casualty rate over a four month period would equal....
