What might cause another 9/11?

Today's defense of President Obama from Andrew Sullivan is devoted to refuting Conor Friedersdorf's criticism of Obama's drone program. Says Sullivan:

What frustrates me about Conor's position - and Greenwald's as well - is that it kind of assumes 9/11 didn't happen or couldn't happen again, and dismisses far too glibly the president's actual responsibility as commander-in-chief to counter these acts of mass terror.

This is exactly backward. I absolutely believe that another 9/11 is possible. A...