"This isn't a court case, they had an iron-clad case for war." The only point of a cassus belli is to justify your war of aggression, if someone declares war on you the case for it is clear.

And no, the cassus belli was not iron-clad. There are only three legal ones under current law. Defense, defense of an ally, and a UN approval. First two don't work, Afghanistan hadn't declared any wars, and the third never happened.