The smartass reply would have been to ask Paul whether, if in fact conditions in Iraq precipitated 9/11, it wasn’t perfectly appropriate then to take extreme measures to try to remedy those conditions afterwards. A more thoughtful response would have been to ask him what his studiously noninterventionist “constitutional” option would have been when Saddam invaded Kuwait. But that’s all gravy; Rudy’s answer suffices as an expression of the palpable disgust most Americans (or at least most conservatives) felt at that moment for that Bircheresque crank, which is why he got the reaction he did. You can hear Mitt at the end over the din demanding that Rudy not be given the extra 30 seconds he requested, and with good reason — he might have walked away with the nomination right there.
I tacked on a bit from the post-debate interview with H&C for good measure. I like the analogy Rudy draws between Paul and another terror apologist he once encountered.