Nuclear war. Iran survives the strike well enough to retaliate by striking Jerusalem with a nuclear weapon. Israel responds in kind with strikes on Tehran and smaller strategic strikes on suspected nuclear sites. The international community intervenes, but not before the death toll rockets into the millions, the economies of Israel and Iran are crippled for the foreseeable future, the effects spread through the region impoverishing much of the Middle East, the US loses its alliances with many members of NATO, and global warming is accelerated significantly. We will have to wait years to know the true extent of the damage.