A House of Dynamite

The title card "Hitting a bullet with a bullet" is shown over a reenactor from Gettysburg. Famously, there is an example of a bullet hitting a bullet and fusing together at the Gettysburg museum. According to legend, this happened during Pickett's Charge on July 3rd, 1863.

In the absence of any threat against the American nuclear deterrent, the nuclear command authority would very definitely put a counterstrike on hold for several hours at least, by default, because in the given scenario, they do not even know which nuclear power is attacking the USA. Once the nuclear event has occurred though, they can then have the fission products analyzed and compared to previous analyses from known sources, previous foreign nuclear tests, fission products in the exhaust of foreign nuclear facilities and such. All this data is on file. It would very probably at least reveal the source of the bomb fuel with great certainty.