Oh, don’t get so cock sure of yourself there. A possibility exists that we could get hit by material that originated from an ally...or even that originated from ourselves.
Then what would we do?
Well, there's a possibility we could all die a fiery death in a meteor impact tomorrow, but I'll keep worrying about car crashes and heart attacks, thanks.
We can distinguish between Chinese, Russian, Pakistani, Iranian and North Korean sources. Those are the only nations I would ever suspect of deliberately giving nuclear materials to a terrorist group to use on us, and among them only Iran or North Korea would be so foolish as to try it under their current regimes. Pakistan might, but only if the government falls into radical Islamist hands. All of those nations are fully aware that there would be severe repercussions if any of their material saw use in a dirty bomb or nuclear detonation.
-ccm