It's not s problem at all. It's actually the whole point.

Following Saddam's botched invasion of Kuwait, the regime was ordered to destroy it's WMD stockpile. The UN was mandated to foresee Saddam's WMD programmes were destroyed. Saddam spent the following years outright preventing the UN to do any form of verification, and went to the extent of outright antagonizing them.

So you reach a point where a totalitarian regime with a long and proven track record of developing and using WMDs refuses to show it got rid of it's WMDs. How can you tell if they still have it if they actively prevent the UN from checking?

You instead receive intel that suggests Saddam is indeed not only stockpiling WMDs but also actively developing them.

Do you think it's unreasonable to enforce the decision?

It's tempting to look back and take the simplistic and ignorant path of saying "there were no WMDs". This however denies all facts and state of affairs. In fact, the whole WMD talk is a red herring.