To get into the 9/11 Memorial Museum, you have to pass through a world-class security arrangement—a conveyor belt for shoes, belt buckles, cell phones; a three-second hands-above-your-head body scan—overseen by a notably grim private-security corps. “Stand there!” uniformed guards shout at those in line moseying ahead. “Don’t advance.” A terrorist planning to commit an atrocity at a museum devoted to the horrors of terrorist atrocities might seem unduly biddable to his enemy’s purpose, but then perhaps the security apparatus is itself a museum installation.