There is no more sacred room for military officers than 2E924 of the Pentagon, a windowless and secure vault where the Joint Chiefs of Staff meet regularly to wrestle with classified matters. Its more common name is “the Tank.” The Tank resembles a small corporate boardroom, with a gleaming golden oak table, leather swivel armchairs and other mid-century stylings. Inside its walls, flag officers observe a reverence and decorum for the wrenching decisions that have been made there. Hanging prominently on one of the walls is The Peacemakers, a painting that depicts an 1865 Civil War strategy session with President...