Basic Rule 1 - terrorists are not actually soldiers
No matter how often people refer to the "war on terror" or "Bin Laden's army of Al Qaida terrorists", one fact remains: when you're a terrorist, you don't play by the army rules. Terrorists aren't representative of their governments, they are only representative of their causes.
You'll note I say "causes", plural. That will be important later.
Basic Rule 2 - The defeat of an army means nothing to a terrorist
See, if you don't report to a government, it doesn't matter much to you whether or not that particular government's army has conceded defeat to your army. Since the government's not paying your check, you don't care who's surrendered to whom, you're still out there fightin'. No governmental ties, no respect for things like treaties, or cease-fires, or surrender. Geneva convention? Don't care!
Basic Rule 3 - There are more terrorist causes in this world than we could hope to identify
Yes, Al Qaida is a major force in the terrorist world, but they are far from the only terrorists that exist. In fact, the smaller your cause, the more likely you are to use things like home-made car bombs driven in to a hotel lobby frequented by people you consider to be oppressing you. Even if we could completely eliminate one large group, we could never hope to identify all the smaller ones, not with all the time or money in the world. It would be like trying to find the bad grains of sand on a beach. Admittedly, it's a small beach on the Mediterranean, but it's still a near impossible task.
So, combine rules 1,2 and 3, and come to the same conclusion that I have - terrorist activity, even if it's funded by a country's government, is not directly controlled by it. Punishment is seen as a type of martyrdom that simply inspires others of the cause to greater heights of violence. That's why arguments like the one on the new Becker-Posner log (nod to Running Scared), let's say Becker's piece on how preventative war can be justified must ultimately be discarded as naive and wrong.
Combating crime mainly relies on deterrence through punishment of criminals who recognize that there is a chance of being apprehended and convicted-the chances are greater for more serious crimes. If convicted, they can expect imprisonment or other punishments- again, punishments are generally more severe for more serious crimes. Apprehension and punishment reduce the gain from crimes; in this way, it deters others from criminal activities.We're not "deterring the criminals" by going to war with regulation armies. The criminals don't care what we do to the armies - wait, no, I take it back - the criminals LOVE when we attack the armies, because it just goes to serve as a great example to the children that they're brainwashing that the U.S is an evil, evil place that's trying to take over the whole world by force.
It is because of the utter lack of rules, and the complete refusal of terrorists throughout the world to operate in a civilized manner or venue, that Yasser Arafat was never able to control the activities of Hamas. It's also why defeating the Iraqi army and ousting Hussein from power has done us no good whatsoever to secure peace in Iraq so far.
This war is not preventing future problems, rather it is opening the door to a never-ending list of problems that we have no expertise or understanding to solve. In my opinion, nothing could ever justify that.