It was all about the WMD's he possessed and thus a threat to the region. He was always a threat to the region and when he was fighting against Iran the USA was happy to support him. When he was using WMD's in the form of chemical weapons against Iran why was there invasion then?

Another country in the area has almost certainly stockpiled a huge arsenal of weapons including nuclear weapons, and hasn't signed the nuclear non-proliferation treaty, therefore doesn't even get inspected.