It’s been five weeks since this impromptu exercise in online teaching about terrorism began, and now we’ve come to the end. So what I’ve asked my students to think about for this final week of class are two simple questions:

• Why hasn’t there been another 9/11-scale terrorist attack in the United States in the nearly 20 years since?
• What are the real risks that terrorism poses to the United States, and how should we address them?

One of the ways I get at that first question is by having my students read a piece by journalist Timothy Noah, published back in 2009, in which he lays out a series of possible explanations for why another 9/11 had not yet occurred. Despite being a decade old now, many of the theories he lays out have held up well, others not so much.

As I discuss in this first video below, Noah’s explanations range from the comforting to the decidedly worrisome. On the comforting side, Noah essentially argues that 9/11 was a fluke that won’t be repeated. Rather than succeeding out of strategic brilliance and flawless tactical execution, the 9/11 attacks worked because of dumb luck. At every one of the many points where the plot could have been discovered or something could have gone wrong, the breaks went in favor of the terrorists. The likelihood of that happening again, he argues, is pretty darn low. On the worrisome side of the equation is the simple argument that another large-scale attack is ultimately inevitable, and it is simply of matter of time until it happens.

Should another large-scale terrorist attack happen in the US, many analysts believe that it will likely involve the use of weapons of mass destruction, or CBRN (chemical-biological-radiological-nuclear) terrorism. Journalist Steven Brill makes this point in assessing US terrorism security policy post-9/11. In the video below I summarize the case made by leading terrorism scholar Bruce Hoffman on CBRN. He touches on three main points: Why we haven’t seen CBRN terrorism yet; why it might now be plausible; and why we shouldn’t dismiss the threat.

In the final video, I introduce a contrarian argument, courtesy of Ohio State political scientist John Mueller, who argues that the risks of terrorism for the United States have been wildly overblown (see a short review of his book by that name here), that our overreaction to the risks is more damaging and dangerous than the threat of terrorism itself, and that rational policy making must be based on a clear-eyed assessment of what terrorists actually can do rather than our “worst case fantasies” about what they might want to do.