For one, we're not "going on the offense" without having been provoked. We've BEEN provoked, over and over again. This is called a RESPONSE. In other words, it is defensive as our intent is not to ATTACK AND ANNEX, as Hussein did to Kuwait, but to PREVENT BEING ATTACKED OR BLACKMAILED, by a regime which has already amply demonstrated its intentions to do BOTH.
But - If we had gone on the "offense" when bin Laden declared war on us in 1997/98, the attacks of September 11, 2001 would never have occured. (And we had good reason to kill bin Laden long before he even declared war, as he was already attacking us even before.)
"The best defense is a good offense."
It is reasonable to assume that if someone uses chemical weapons on his own people, that he has absolutely no scruples and has absolutely no reason to go on living. Do you want to take a chance in an age when playing "defense" means you have to wait until the bad guy sets off a nuke in your city, or until the water supply is poisoned, or until a genetically engineered disease is spread, or...?
We're not asking to bomb England, Tahiti or Ethiopia, for Pete's sake- we going after a regime whose own citizens desperately want their leader dead. A regime which has shown a desire to ANNEX other people's countries, a regime which has stated its desire to destroy another country entirely, a regime which has thought it good to set alight oil fires to estroy the precious resources of another nation and a regime which has tried to poison the water of other nations by pumping oil into the sea and endangering their desalinization plants, a regime which launched scud missiles on the civilians of a neutral country, a regime which carries out routine assassinations of its dissidents overseas, a regime which assassinates its own scientists to prevent them from talking, a regime which harbors and protects as well as uses known terrorist groups, a regime which is actively seeking nuclear capability so as to invade other nations and then blackmail others with the threatr of using those same weapons so as to be left alone to absorb its vlctim.
We really do not want to see Hussein become another North Korea... we really do not want to fight this guy again on his terms a few years from now when he finally has an arsenal of nukes. It is better to fight him on our terms and be rid of him, so Iraqis and Americans and al those whom Iraq has been threatening and bullying can get back to work and enjoy life.