Whatever you believe about any of those things, you should disregard what appears to be President Obama’s chief case for nominating him: that he served honorably as a sergeant in Vietnam, where he was twice wounded in combat. …

What is it, precisely, that one would bring by service as a sergeant in a war more than 40 years past — almost as distant from today as the charge up San Juan Hill was from D-Day, or the Battle of New Orleans was from Gettysburg? It was an important, even searing, life experience, no doubt. But the technology, strategy, tactics and organization now are all utterly different. Today, we have a hardened professional army, not a band of reluctant conscripts caught up in the Big Green Machine. And a defense secretary is not the secretary of the Army: The other services have very different equipment, cultures and problems. …

There are plenty of sergeants in this world — good, bad and indifferent; wounded and whole. They are the backbone of the armed forces. But their experiences and responsibilities are not those of the secretary of defense. He or she must wrestle with one of the world’s largest bureaucracies; make difficult choices among extraordinarily expensive technologies; show discrimination and judgment in picking and, if necessary, firing generals; balance domestic and foreign politics; knit his or her department into the intricate web of interagency relationships; and advise wisely on strategy and campaign plans.