In the dis-United States, nothing!
Committees would be formed, bluster would cloud the airwaves, the Ruskies would say they didn't do it, and more tyranny would be imposed on the American serfs by our benevolent communist rulers...
A nuke going off on European soil will devastate the Eurozone economy. No banks will loan any money. These effects will reverberate over here.
At the very least, I'd expect a run on the grocery stores, wiping out the shelves.