“How many jets can you get airborne?” It was NORAD, calling the 122nd Fighter Squadron in New Orleans on the morning of September 11, 2001. “We’re loading missiles on jets now,” I said. “We’ve only got four pilots.” Five minutes later, the phone rang again. “Launch everything you can get airborne and find Air Force One.” We scrambled four F-15s and headed for Barksdale Air Force Base, where President Bush had just addressed the nation. Unrestricted climb to 23,000 feet. “Houston Center, Jazz 1 requests 44,000 feet and a heading of 330.” “Any altitude, any heading, Jazz. The airspace is yours.” During the climb, Houston Center came back on the radio. “There’s only one other plane airborne, and I suspect that’s who you’re looking for.” After that, the radio was eerily quiet as we sped toward northwest Louisiana. We got radar hits on Air Force One, but were still miles away when we received word that a flight of F-16s had arrived to escort the President. The rules of engagement we were operating under that day have since been declassified. They were simple and sobering: The Vice President has cleared us to intercept Targets of Interest and shoot them down if they do not respond. Think about that for a moment. For generations, geography had protected America. Two oceans separated us from our enemies, and we trained to keep threats from getting here. On September 11, that all changed. The enemy was already inside our airspace, and the aircraft themselves had become the weapons. That was the day our definition of homeland defense changed. We can’t assume the next threat will look like the last one. We can’t assume distance will protect us. And we can’t afford to become so comfortable that we forget what we’re supposed to be watching for. September 11 isn’t just about remembering what happened. It’s about remembering why we have to remain ready.