This has always bothered me. What will we do? What if we lose several cities? Do respond with nukes? Where do we nuke? Scary.
The biggest problem would be the economic collapse. No one who works in a US city would ever feel safe going back to work. The stock market would crash hard - 1929-style. Europe would line up against us, rationalizing that we deserved it and hoping that by speaking against us they wouldn't be the next target.
In short, any nuclear attack on any US city could destroy the United States as a functional state within a few years. Thanks to the NEA, we aren't the fierce, resilient, determined people we were in the 50's - look how we've pretty much forgotten 9/11 already. Al Qaeda knows this, and I'm sure the US Government does, too. Interesting things must be happening behind the scenes.
Washinton D.C. is the seat of corruption in America, and without all of the support that communists get from our socialist pinko democrat representatives, their power would be gone. This is starting to get scary as hell, and frankly if we are nuked, we need to respond with extreme predjudice. If it's a bunch of Muslims, we nuke Mecca. Just the threat alone should keep them from starting anything with us. I think we need to let them know in a not so subtle way that any nuclear action against us will end in the terrorist country's extinction.