There isn't one, that's the point! Remember way back when that one kid in the neighborhood didn't get his way and takes his ball and goes him? He didn't get to play either.
When the possum can't roll over and play dead, he hisses and shows you a quite formidable mouth full of teeth.
He will not win the fight but he sure will make you pay for his death.
There is always a possibility of a Total Global Thermonuclear War first strike. It will not make sense to anyone except the person who pushes the button but anyone that thinks it cannot happen is a fool.
Hint: there are people out there who think, why have nuclear weapons if you don't use them.
Another hint: There are even more who think they can survive a Total Global Thermonuclear War, just for the opportunity to make a fresh start.
If I didn't have a wife of fifty years and a daughter to think of, I might be one of those guys.
Did you ever read Farnham's Freehold? It's sort of like Galt's Gulch with a Nuclear War and some Islamic hegemony that took over in the future.