An interesting thought experiment:

What Would Happen If the 20 Biggest US Cities Were Wiped Out With Nukes

In short: Jihadists wander off with 20 45-kiloton Pakistani nukes and detonate them in 20 carefully chosen major US cities. The end result is million dead, the relocation of the Capitol to Denver, the collapse of the US economy, a series of Constitutional crises over succession after much of the chain of command is wiped out, a US military response that takes out a chunk of Pakistan.

The variables are too many to guess at, but I could see the US military response to such a situation range anywhere from virtually nil, to lashing out with a whole bunch of Trident missiles raining nukyular death all over the middle east.