The best argument against the War on Terrorism is that terrorism simply isn't enough of a threat to be worth the resources we're pouring into it. On the balance, in a universe that permits things like nuclear weapons, I'm not quite willing to bet the farm on that argument, but I certainly think it's well within the field of factually-viable opinions on the issue.
Bolstering the "terrorists really can't be large-scale threats" theory is this article from the Wall Street Journal OpinionJournal, Iran's Economic Crisis, which I will quote selectively and extensively from: