By January 2012, the State Department will do something it’s never done before: command a mercenary army the size of a heavy combat brigade. That’s the plan to provide security for its diplomats in Iraq once the U.S. military withdraws. And no one outside State knows anything more, as the department has gone to war with its independent government watchdog to keep its plan a secret.

Stuart Bowen, the Special Inspector General for Iraq Reconstruction (SIGIR), is essentially in the dark about one of the most complex and dangerous endeavors the State Department has ever undertaken, one with huge...