Eliot A. Cohen teaches at Johns Hopkins University’s School of Advanced International Studies and is the author of “Conquered Into Liberty: Two Centuries of Battles Along the Great Warpath That Made the American Way of War.”

You may like the idea of Chuck Hagel as defense secretary or loathe it. You may consider his views on Iran sound or feeble, his comments about “the Jewish lobby” inoffensive or ugly, his views on a policy of extensive assassination — sorry, “taking terrorists off the battlefield” — unremarkable or chilling, his apology for harsh remarks about a gay ambassador sincere or opportunistic. 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.

That is not, in any way whatsoever, to minimize Hagel’s service to this country or that of any other veteran in that long-ago war. Rather, it is to say that military service has very little bearing on the effectiveness of the second most important civilian leader of the armed forces. Our two most effective wartime presidents, Abraham Lincoln and Franklin Roosevelt, had virtually no military experience. Jefferson Davis, by way of contrast, in addition to betraying the Union, was a fine officer, a veteran of the Mexican War and a president who could barely stay on civil terms with any general other than Robert E. Lee, who managed him rather than the other way around.

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.

Does combat service uniquely produce empathy with the troops, an awareness of the horrors of wounds and violent death? Visits to a military hospital will bring one to that. Did Defense Secretary Bob Gates care any less about the troops because he wasn’t hit by shrapnel during his Air Force service?

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.

And — the hard part — the defense secretary must be quite capable of sending young men and women into harm’s way. If he or she cannot do that and still sleep well at night, he or she has no business being in that job. The head of the Department of Veterans Affairs must “care for him who shall have borne the battle and for his widow and his orphan,” but it is the duty of the defense secretary to prepare for war, to make sure that our arms are the best, our servicemen and women not merely well-trained but keen, and our face to the world one of confidence in the power of our arms. If one hopes to deter war, or the things that lead to war (an Iranian nuclear weapon, for example), that is the visage a defense secretary needs.

Vietnam still casts a pall over our public discourse. Veterans who justly resent the treatment they got when they came back and the mishandling of the war may square off against those who avoided the draft and, as a result, cringe or bluster or simply shrink away from dispute. The rest of us, including those of us too young to have gone, much less take part in the decisions that shaped the war, look at the veterans chiefly as victims, committing thereby an injustice to the soldiers and, in some measure, to the cause for which they fought.

The job of defense secretary is, the management expert Peter Drucker once said, impossible. That’s probably true: The bureaucracy is so complex, the intellectual challenges so elaborate and the pressures so heavy. This is all the more reason not to think of an appointment to that position as a means of doing justice to the grunts, or a scene in a larger morality play of American history.

Read more on this debate:

Aaron David Miller: 5 myths about Chuck Hagel

Dana Milbank: Battle-tested for a fight

Before he can become Secretary of Defense, Chuck Hagel will likely face sharp questioning in his upcoming confirmation hearings. Post columnist David Ignatius says the confirmation battle could serve as a preview of the most important foriegn policy debates of 2013. (The Fold/The Washington Post)

Carter Eskew: How Vietnam affected Chuck Hagel

Jennifer Rubin: Old soldiers don’t make for good Pentagon chiefs