Is war ever necessary? In a limited sense, sometimes yes. If you are attacked, you have the right (and the need) to defend yourself. That applies equally well to individuals and groups, including nations. That's probably the only time war is necessary, when survival demands it (such as when Germany invaded Russia in 1941). But there is also the notion of the just war: this includes self-defense but also includes defense of an ally. In that sense, while the Gulf War was not necessary to the coalition ranged against Iraq, it was just because it was to liberate Kuwait which had been invaded and annexed by Iraq.
In the present case war seems neither necessary nor just. No one is being directly threatened by Iraq; the expressed motive is to rid Iraq of WMDs. It is being justified on the basis of pre-emption. The sticky part is, that while many nations posess weapons of mass destruction few have ever used them. Only two low-yield atomic devices have ever been detonated in warfare; chemical weapons had only general usage in WWI; the use of biological weapons is a little bit iffy. Most countries shun WMDs, even those which have them. One exception has been Iraq. It has used chemical weapons during the Iran-Iraq war (and against its own subjects at other times), may have used biological weapons during the Gulf War, and is known to have been actively pursuing projects to 'improve' on these weapons and pair them with long-range missiles. Does this justify pre-emption? Probably not. Granted, Iraq could if left alone create devices that could create wide-spread death and devestation with ease, but just as it would be unthinkable to execute a man because you think he might commit murder, I don't think you can attack a country because it might be developing WMDs. Containment seems a far saner approach.