The bombs fell, and civilization was wiped away. All that remains is rubble, dead and decaying. No electricity, no organized industry, barely any medical care. All that remains is the day-to-day struggle to fight.
But what can one fight for? Why fight for land when it's either burned, bombed, or polluted? Why fight for money when there's nothing left to spend it on? Why fight for natural resources when they're all put to use fighting? Why fight for religion when it has no more meaning?
Why fight at all?
For months or years, the Armies have been asking these questions. And when all of these facts are realized, the only reason left is the most logical one of all.
To win the war.