They don't need another 'cause' to rally around. They already have half a dozen, everything from Israel's existence to American culture. The real problem is that they will use nukes on the US and then what will we do? Demand an apology? Take our grievances to our friends at the UN? Anything other than the use of overwhelming force will be looked on by the Muslims as weakness and will encourage the use of more WMD's against us. We will end up like the black knight in 'Monty Python and the Holy Grail', chopped to bits, but still yelling our defiance.
I'll ask you, too: who is this "they" you're talking about?