Fifteen years ago this September 11, 19 terrorists, using four jetliners as guided missiles, killed 2,977 people—and enveloped the country in fear. It was the first sustained attack on American soil since the bombing of Pearl Harbor, which was a far-off military base. This massacre hit the center of our government and blasted away part of our most iconic skyline. It left a stench that New Yorkers could smell weeks later as remains continued to be recovered from the ashes.

Suddenly, we were vulnerable. Not just to disease, tornadoes, accidents, or criminals, but to the kinds of enemies that had always threatened others but never us.

Barack Obama remembers that after the second plane hit, he left the Chicago building that housed his state-Senate office. “I stood in the street and looked up at the Sears Tower, fearing it might be a target, too,” he told me in a recent email exchange, adding, “I remember rocking Sasha to sleep that night, wondering what kind of world our daughters were going to grow up in.” He continued, “With nearly 3,000 people killed in the places where we lived our daily […]