The Pentagon is the headquarters of the single largest institution in America: the Department of Defense. The D.O.D. employs millions of Americans. It owns and operates more real estate, and spends more money, than any other entity. It manages the world’s largest and most complex information network and performs more R&D than Apple, Google, and Microsoft combined. Most important, the policies it carries out, in war and peace, impact the security and freedom of billions of people around the globe. Yet to most Americans, the dealings of the D.O.D. are a mystery, and the Pentagon nothing more than an opaque five-sided box that they regard with a mixture of awe and suspicion.