unpatriotized history:

The US government was not the authority to make rulings about which countries had certain weapons, nor was the US government the authority to take action about any such weapons in other countries. The US government was forbidden from attacking other countries regardless of what it thought or when it thought it.

In 2003, international law did not give the US or any other country the authority to make determinations about who had weapons, and it forbade all countries from attacking other countries except in self-defense if an armed attack had occurred.

If you have legal authority over an area, then the legality of your violent actions against people in that area might depend upon what you knew about them and when you knew it. But if you do not have legal authority over an area, and you still took violent action against people in that area, then it makes no difference what you knew or when you knew it.

International law did not give the US the authority to determine who had certain weapons or to attack those who did.