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Much of a defense secretary’s work is at his own discretion. He is responsible for military budgets and procurement, personnel promotions, public diplomacy, the Pentagon’s relations with defense ministries and militaries around the world, tactical military movements, and most force deployments. When a commander asks for an additional unit or capability—as with Les Aspin in Somalia, with Donald Rumsfeld and Robert Gates in Iraq and Afghanistan, and even with Leon Panetta in Benghazi—the request lands on the secretary’s desk. And more often than not, it is the secretary, not the president, who makes the call. Such heavy responsibility is in the nature of the job. The secretary of defense is the second in the military chain of command (below the president...