If you sell someone a gun, and he shoots someone with it, does that mean we can't punish him? Does it even mean the person who sold him the gun can't punish the killer? No, it only means we can punish both. And sure, selling WMD to Iraq was a big mistake, but it doesn't justify the use of WMD. We still have the right to wipe any sucker off the face of the Earth.

Edit: Maybe I misunderstood something here. Are we discussing whether the US govt is stupid, or are we trying to /un/justify war? If it is the former, I've got nothing to say to its defense