re: good argument..
I doubt that it will prove out to have been a "major" clearing house for terrorists but it seems to have become one. If it has then it is a good place to fight them.
The argument against military adventurism is also a good one. It will take on more or less validity depending on the outcome.
If we fail in Iraq it will probably be the last pre-emptive action undertaken but I fear that the ultimate cost in lives for both sides will be far greater if it turns out that way.