"We went to war with the assurance that stockpiles of chemical, biological and possibly nuclear weapons were in Iraq, with no maybes."

Have reference which supports this statement?

What I recall is that there were hundreds of tons unaccounted material that could have been made into chemical & biological weapons. That Saddam had the motive and means of making WMD. And that Saddam had the UN inspector thrown out of Iraq in '98.
I remember US soldiers wearing chem suit in hot weather because there was a possibity that Saddam could use chemical weapons against our troops. And also that Iraq battlefield commanders thought other Iraqi commanders had and might use chemical weapons. We knew where there were large ammo depot areas were. And we generally knew where the artillery which might use chemical weapons was located.

So before the actual start of the miltary operation to invade Iraq who is giving "assurance that stockpiles of chemical, biological and possibly nuclear weapons were in Iraq, with no maybes." ?

What I recalled hearing was a lot of terms like "potential", "could have", "what is believed by some" with main theme being that there a fair amount of uncertainty and there was the danger that we could be underestimating the threat. In other words there was a lot of maybes.