President Obama came into office selling the American people on a fiction: that we could quickly wind down the offensive fight, roll back the defensive security state, and we’d still be safe. This fiction couldn’t survive security briefings. So what did he do? He left the Bush defensive infrastructure in place while winding down our commitment to offensive military operations (with the notable exception of pinprick drone strikes). We wound down our offense without inflicting the damage we should have inflicted (in Afghanistan, primarily) on an announced timetable, and without leaving a residual force (in Iraq) or perhaps sufficient residual force (in Afghanistan). These factors give our wounded enemy an opportunity to hunker down, wait for our departure, and then face much weaker forces when it re-emerges in force and attempts to establish the kinds of safe havens and training camps that render it exponentially more dangerous.
If this happens, then our risks skyrocket, and our defensive capabilities become that much more important. He has to know this. Anyone with the slightest familiarity with our enemy has to know this.