The Pentagon wants to buy every single copy of the first print run of Operation Dark Heart, an expose of operations in Afghanistan and Pakistan by an Army intelligence officer. Now why would they want to go and do that?
Well, if you ask them, Operation Dark Heart contains "highly classified material"—not just stories of secret missions but also the names and identities of intelligence agents. But the publishers say they've removed the sensitive material—and that at this point the Pentagon is asking them to take out "information that is 20 years old or even in the public domain."
So the speculation, therefore, is that the military brass is more concerned with its own negative portrayal in the book than with the actual "secrets" contained within. Operation Dark Heart is written by Lt. Col. Anthony Schaffer (also known as "Christopher Stryker"), who's been a vocal critic of the U.S. Defense Intelligence Agency (which he claims ignored evidence that could have helped prevent the 9/11 attacks), and it's not going to be very kind to the Pentagon. (Schaffer, still a reserve officer, is being blocked by the Pentagon from speaking publicly about the controversy.)
Not that St. Martin's Press is complaining about the fact that someone wants to buy every single copy of this book! (Nor, for that matter, are they complaining about the free, awesome publicity that accompanies the whole ordeal.) At this point, anyone buying a book is nice, really, even if it's just headed for the big basement where they keep the Ark of the Covenant.