"We should go to war because arms inspections don't work, based upon their failure in the mid-nineties."
What failure are you referring to, specifically?
"From 1994 to 1998 Iraq was subjected to a strenuous program of ongoing monitoring of industrial and research facilities" .... "It allowed UNSCOM to ascertain, with a high level of confidence, that Iraq was not rebuilding its prohibited weapons programs and that it lacked the means to do so without an infusion of advanced technology and a significant investment of time and money."
"Verifying Iraq's complete disarmament was complicated by the fact that in the summer of 1991 Iraq, disregarding its obligation to submit a complete declaration of its WMD programs, undertook a systematic program of 'unilateral destruction,' disposing of munitions, components, and production equipment related to all categories of WMD. When Iraq admitted this to UNSCOM, it claimed it had no documentation to prove its professed destruction."
"While UNSCOM was able to verify that Iraq had in fact destroyed significant quantities of WMD-related material, without any documents or other hard evidence, it was impossible to confirm Iraq's assertions that it had disposed of all its weapons. UNSCOM's quantitative mandate had become a trap. However, through its extensive investigations, UNSCOM was able to ensure that the vast majority of Iraq's WMD arsenal, along with the means to produce such weaponry, was eliminated."
"As such, the world found itself in a situation where the considerable accomplishments of the UNSCOM weapons inspectors�the elimination of entire categories of WMD and their means of production�were ignored in light of UNSCOM's inability to verify that every aspect of these programs was fully accounted for."
In other words, it cannot be known is whether Iraq still has something left in their arsenal. The U.S., however, has failed to make public any convincing evidence to the contrary, and there is no way to prove if Iraq does still posess weapons of mass destruction, short of either further inspections or war. You can't prove a negative.
The one thing the inspectors agree on is that Iraq lacks the capability of getting any stronger. All inspections can do is further reduce his arsenal. However, if Iraq does have any WMDs left still, an invasion would clearly give him a reason to use them.