No first use.
No use a-tall until you definitively identify the source of the attack.
Which essentially means no use ever.
Better just cowboy up because someday, someway we're going to get hit and there will be no clear culprit to hit back.
We're just going to take a few on the chin and have to overcome that and move on.
(And get it through your heads that "nuclear forensics" WILL NOT identify the source of the uranium every time. It might if you have "known source" samples to compare against, but you don't have that for Iran, NK, Pakistan and probably some others. Beyond that, if it definitively proved out to be sourced by the DirkaDirkastanians, the DirkaDirkastanians will claim the material was stolen from them years ago.)