There is no single institution in America as big as the Department of Defense. It spends more money than any other. It employs millions of Americans. It owns and operates more real estate than any other entity. It manages the world's largest and most complex information network and performs more scientific research by far than any other institution. For all these reasons, the Pentagon is a vast and powerful presence in our national life. Yet to most Americans, it is mysterious—a largely opaque five-sided box that many regard with a mixture of awe and suspicion. The need for this book, which demystifies the Pentagon and lays bare the inner workings of Department of Defense, is more urgent than ever. And there is no person alive better suited to telling it than the 25th Secretary of Defense, Ash Carter.