Yes, I very definitely mean pre-airport intelligence, but it's far more than wiretaps or email monitoring. The required operational security (opsec) or a mission that requires only one person is extremely low. Change that to five people and you now have not only the electronic communication between those five people, but physical contact, a much larger command-and-control structure, and ways to get funding. The opsec requirements for a five-person mission are far higher than for one person.

Terrorists aren't a special forces group. People who operate at that level of the organizations aren't usually even very bright. They don't tend to do opsec very well. That means there are far more opportunities for them to make a mistake that will lead to their being caught or for the group to be noticed.

I think most people here would agree that we can't have a situation where the checkpoints are the major barrier to a successful terrorist operation because the required level of security is impractical. Instead, the purpose of checkpoint security is to deter single-person missions (which is why we don't see anybody "caught") and make it require enough people that we have a reasonable chance of catching them in other ways.

Yes, that's not foolproof. But I think it's the best we can do as a nation if we want to retain civil liberties.