I answered "Iraq/Iran" but really mean the greater Muslim world. If the current political climate were to yield a world war, which I doubt, I believe the escalation would be as follows. Islamic extremists achieve significant recruitment, possibly accomplishing an attack on the level of 2001 in the US or one of our allies. We and allies respond by operating in Islamic nations without consent of the relevant governments. Those Islamic nations enter the fray. The world politic divides into those concerned with terrorism against those concerned with the sovereignty of the relevant Islamic nations. Everybody dies.