I am not a missileer. But, I have heard enough lectures and read enough in my studies to have an inkling.
I think the method of the act would dictate our response. In the case of a ground burst with no apparent outside delivery, I think we would not just let “all birds fly”. If we did that, we would basically hit a target which might not be the actual target, and would leave us open and vulnerable to another attack. In this case, there’d be an investigation before a reaction.
Now it’s no secret that during the Cold War, we expected the Soviets to bomb us with ICBMs. We have early warning stations still operating to at least tell us they’re coming, and from who. Now in that case, we know what’s going on. Thus, we have a no-nonsense policy of responding immediately and with full force. Back in the 60s and 70s it was called “Mutually Assured Destruction” [MAD]: you pee in my Corn Flakes, and I’ll knock your Alpha Bits outta the milk.
That’s also where the whole Single Integrated Operating Plan [SIOP] was born: to give the National Command Authorities several plans in case of a no-notice nuclear attack. Many plans and scenarios were put together, but I don’t know if a “suitcase nuke” was one of 'em.
My answer: We’d wait to find out who did it, then give the perps two or three sunrises in one day.
“. . . load up, take off, and nuke it from orbit. It’s the only way to be sure.”