Here’s a question I found myself batting around with Yglesias last week: How many committed al Qaeda operatives, people willing to kill and perhaps die for the cause, do we think there are in the U.S. right now? The initial New York Times piece on the NSA wiretap story suggested that about 500 people were being tapped at any given time. How many do we think were full-blown al Qaeda terrorists? One in ten? One in twenty?

I wonder, because if you’ve ever engaged in the rather morbid thought experiment of contemplating what it would take to stage some catastrophic and deadly attack, it actually seems terrifyingly easy. It would take a fair amount of work and planning, of course, but 9/11 was pulled off by a relatively small team on a relatively small budget, and it certainly seems like you could do a sub-9/11 scale but still highly destructive attack with a lot less—a couple guys, a rented truck, and some explosives, say. And if you were an al Qaeda member in the U.S. in the years following 9/11, mightn’t it seem as though the newly aggressive efforts to track folk like you down meant it was advisable to get anything you were planning executed as quickly as possible?

Maybe we’ve just been that effective at catching these folk—or maybe we managed to deport a big chunk on visa violations—and I’m certainly not implying there aren’t any U.S.-based al Qaeda. But if they’ve got even a fraction of 500 people here involved in their plots, why hasn’t one of them managed to pull anything off?

—posted by Julian