but the US knew Saddam had chemical weapons

Yes, I know... the US knew about every gallon of mustard gas and sarin Saddam had - not only did the US provide intelligence to Saddam to specifically help his use of the stuff during the Iran-Iraq War be more effective, they could also literally read almost every damn ciphered communique of the Iraqi army at the highest levels thanks to the fact that Iraq used compromised ciphering machines bought from the CIA front company Crypto AG in Switzerland.

They knew perfectly well Saddam had destroyed all his stockpiles - if he hadn't, Bush would have never dared to invade Iraq.