I found this 2011 interview with Stephen Hadley recently. Here’s what he said about the absence of WMD in Iraq:

…it’s the question my daughter asked me, “Daddy, if you knew before you decided to go to war that in fact Saddam had destroyed all the stocks in WMD, would you have still gone to war?” It’s a real interesting question. My answer, which you’ll think is cop-out, is, “I don’t think the President could have.“

I think if it had been known that there weren’t stocks of WMD, I think those people who would have said try another way, play this out longer, see if there is another way to get this done, would probably have won out politically. I think it would have been very hard for the President to do it. Would that have been the right course? I think it’s very dangerous because our view was that the French and the Russians for various reasons were clearly getting ready to get rid of the sanctions. Because Saddam had basically won the public relations argument and convinced everybody that sanctions were what was impoverishing the Iraqi people, when it was in fact his decisions about how to allocate the money.

I think he would have been out from under the sanctions and I think he would have been back in the business of making weapons of mass destruction, particularly once it became clear that the Iranians were in the business of getting a nuclear weapon. The idea that Iran would be allowed to pursue its nuclear weapons and Saddam Hussein would not be developing a companion piece I think is implausible, particularly since Saddam said that the reason he didn’t tell us he didn’t have any WMDs is he was worried about the Iranians. Can you imagine what he would be doing, given [Ali] Khamenei and [Mahmoud] Ahmadinejad and their nuclear program? We’d be in a nuclear arms race between Iran and Iraq.