Just about the only place in the United States where you saw substantial opposition to the Afghanistan War back in the day was on college campuses. That, conveniently enough, is exactly where I was at the time, so I got to participate in a lot of arguments on this subject. One thing I'm fairly sure absolutely nobody ever pitched to me was "well, don't you see that if we invade Afghanistan we're just going to wind up failing to achieve any of our key strategic objectives because the administration will divert crucial resources and attention to invade Iraq instead?"
That's true in the specific but not true in a more general sense. Opposition to the war in Afghanistan, to the extent that it existed, was premised on the notion that we'd go kill a bunch of people, not help the country afterwards, and ultimately not achieve any strategic goals. Perhaps no one predicted that it would be Iraq that would be the shiny new object which would divert resources, but it certainly wasn't unreasonable to imagine that for a variety of reasons the Bush administration's commitment to reconstruction and aid in Afghanistan would be less than complete.
I'm sure that in the aftermath of 9/11 most people who were less than enthusiastic about the war had a somewhat different body count calculus than those who supported it, placing a wee bit more emphasis on the lives of potential innocent civilian casualties than was allowable in our elite discourse at the time, but the point is that with hindsight it's rather clear that such people should have been listened to a bit more.
Discussions of the utility of the conflict always took a backseat to the perceived moral righteousness of it. Yes we were attacked. Yes that gave us the "right" to do "something" and perhaps something which involved civilian casualties. But, ultimately, we must look back and ask: what did we achieve? At what cost (to us and to others)? Was there a better way?
As has been the case for some many things these past years the choices were never "nothing" or "Pony plan." The choices were always "nothing" or "George Bush's plan." The failure to comprehend that simple fact has prevented members of our very serious crowd of pundits from listening to or admitting to the validity of criticism of so many things. Years later, opponents of the Iraq invasion are almost entirely absent from our mainstream our discourse even though they were the ones who were pointing out what was going wrong in real time even as the Weekly Standard cheerleaders were simply telling us that hope was a plan and that clapping louder was the best thing we could do.
For years it's been a verbal tic of many Iraq war opponents to assert "I supported the war in Afghanistan..." as a necessary prophylactic to charges of "unserious peacenik dirty fucking hippie!" The question is dangling, however... "should you have?" At the very least, shouldn't you have tried to open the door to critics who were less than supportive, not because they hate America, but because they were concerned that George Bush would fuck the whole thing up? Because it was hard to imagine that they'd actually go in and rebuild the place?