Before the Second World War, transparent government was a proud tradition in the United States. But since then, we have radically departed from that open tradition, allowing intelligence agencies, black sites, and secret laboratories to grow unchecked. Officials insist that only secrecy can keep us safe, but its true costs have gone unacknowledged for too long. Using the latest techniques in data science, historian Matthew Connelly analyzes a vast trove of state secrets to unearth not only what the government really does not want us to know, but why. What results is an astonishing study of power: of the greed that it enables, of the negligence that it protects, and of what we lose as citizens when our leaders cannot be held to account. A crucial examination of the self-defeating nature of secrecy and the dire state of our nation's archives, The Declassification Engine is a powerful reminder of the importance of preserving the past so that we may secure our future.