Tom Nichols at The Atlantic:
Some documents, such as war plans or technical specifications about weapons systems, have obvious intelligence value. Most classified materials, however, just aren’t all that sexy at first glance. They’re not likely to look anything like the “Non-Official Cover” list of spies from Mission Impossible; they’re not nuclear codes; they’re not pictures of foreign leaders in flagrante. That type of information exists, of course, but such materials typically have a short shelf life. (Codes and plans, for example, change regularly.) This is why spy agencies, including ours, try to develop assets who can deliver a stream of information over time.
Nonetheless, there are plenty of things you can lug in a box that would be deeply dangerous to U.S. national security.
Technical and scientific documents, for instance, are almost always highly valuable. Enemy engineers can figure out a lot from technical secrets, including how to close gaps with the United States in weapons development, which in turn can create direct threats to U.S. military forces.
There are also other, less obvious threats to security than “here’s where our tanks are parked and here’s how accurate our missiles are.” This is why you often hear the phrase sources and methods when discussing classified information: One of the greatest risks is that an adversary will learn how we’ve discovered their secrets. If a classified document provides the Reese’s Pieces that would help a foreign intelligence organization backtrack and reconstruct how we know what we know, we could lose the sources—technological and human—that provided the material, which is a much deeper long-term problem. On occasion, revealing such sources can result in embarrassment or a minor scandal, perhaps involving a diplomat or some other friendly source in a foreign nation. Sometimes the consequences are much worse.
