That is, they opposed the war on Iraq because they considered stopping that war a necessary step in combatting terrorism. Sure, there were lots of other reasons to oppose that war, but that one was good enough and one which was embraced by plenty of "serious" people.
Or, at least, take the issue up with noted Afghanistan war opposition leader Donald Rumsfeld.
(yes, the whole conservatarian "serious people believe..." construction is one of their most annoying little rhetorical tricks)