Iraq was invaded in 2003 under the official pretense that Hussein had weapons of mass destruction. Well it turned out that he had none which means either the US's intelligence is so shit that they found something that wasn't there or that US intellgience had lied about what they knew. Either way, this makes the invasion of Iraq purely stupid and illegal - the justification of invading Iraq was totally unfounded.
Don't be a fool.
First option is the chemical weapons were moved into some hole in the ground out in nowhere-land desert and the guys that did it killed on purpose by the regime or just through ordinary action in war. This is unlikely, but could have been done if an appropriate way to make it look like something else during spy satellite image processing.
Number two is that they never had those weapons to any appreciable degree, but the cronies told other cronies they had them, so there was a widespread belief that they existed. How does a spy tell the difference between information about chemical weapons that is true, and information about chemical weapons that is false but believed to be true? Saddam's underlings were weaving an extensive lie to him to stay alive.
Also. They DID have those weapons and used them against Iran. USA SOLD THEM to him.
Saddam having those weapons was entirely plausible at the onset of the war.
True, it was probably a bunch of Bush dick waving and a big distraction from the real problem (Afghanistan, Pakistan, and other places with islamists gathering strength) however your conclusion it was all about false pretenses for chemical weapons isn't a sound one.