The war will be triggered by some event that ignites some kinetic conflict over some random issue, of which there are several candidates (e.g. resource competition, nuclear terrorism, the failure of a nuclear state).

But, once started, the combustion will be fanned by the irreconcilable conflict between the "Western world" and the "Muslim world". With modern, worldwide, instantaneous communication, Western decadence simply cannot be tolerated by the Muslim world, which it will inexorably corrupt and destroy. With modern, nuclear, weapons, this conflict will explode out of control and destroy all of civilization, both that in the West, and that in the Muslim world.