They obviously were WMD's there at some point. I don't know if he dumped them just before the invasion or a few years before.
why dump them just before the invasion and not use them during the fighting?
The same reason that they were not used in the 1991 Gulf war (or in WW II for that matter), massive retaliation.
Look at where WMD's (and yes, the US
has used them), have been uses.
1. WW I, both sides in order to maintain the strategic balance.
2. Ethiopia (1936?), Iraqi Kurdistand (1980's), Iran-Iraq War (1980's). Here, only the side using it was the side that had it.
Because it would not have improve Iraq's strategic position to use it, we didn't option one. The US regards all WMD's as basically egual and
may have responded with nuclear weapons.
There is a third case when WMD's are used, as asymmetrical weapons.