But I can’t shake another, darker, question. What if we get hit again with a 9/11-sized attack? More to the point, hypothetically, would a crisis pull us together or drive us apart? It’s a morbid question worth asking before the worst happens, because there’s reason to worry about the durability of what Lincoln called “the better angels of our nature.”

What can we learn from the Bush era? Well, the nation immediately rallied behind the fledgling president (Bush had been in office only about seven months). Members of Congress famously locked arms on the East Front steps of the Capitol and sang “God Bless America.” Bush’s approval ratings soared to 90 percent, as he ordered U.S. troops into Afghanistan to defeat the Taliban and hunt for Osama bin Laden.

The kumbaya period didn’t last long. Three months after the attacks, Democrats mildly questioned parts of the USA Patriot Act, and Attorney General John Ashcroft said such questions “erode our national unity and diminish our resolve.” Time has proved the legislation to be excessive.