President-elect Donald Trump’s nomination of retired Gen. James Mattis for secretary of defense is a good choice. Mattis has a strategic mind; he’s very well-read (many have noted his library of 7,000 books, most of them on history and war); he was an able, clever commander in the Iraq and Afghanistan wars; he’s deeply concerned about the needs of the fighters in the field; and—like many modern generals—he doesn’t fit the nastier stereotypes of the crusty warhorse that he is.

It seems Trump was initially attracted to Mattis for his nicknames—“Mad Dog,” “Chaos,” “Warrior Monk.” Referring to his appointments at his rally in Cincinnati Thursday night, he boasted, “I am going to be putting on the greatest killers you’ve ever seen.” That may be one way of describing Mattis, but it’s far from the whole story. Yes, in one of his many quotable remarks, Mattis told his Marines in Iraq (who all revered him), “Be polite, be professional, but have a plan to kill everybody you meet.” But hey, that’s what Marines do, and some of his fellow officers wouldn’t have bothered with the part about being polite and professional.

More broadly, it’s now well-known that when Trump asked him for his views on waterboarding, Mattis said it was no good. He could coax more information from a detainee with a pack of cigarettes and a couple of beers. Mattis once testified that if Congress cut the budget for the State department, it should boost the budget for bullets because he’s going to need them—and he’d rather address a problem with diplomacy first. Unlike his future commander in chief, he’s also very keen on working with allies, has said it’s the only way to get something done, especially in the Middle East, which has been his main zone of operations.

But this immersion, as a Marine commander, in the wars of the Middle East leads us to some of the problems with this appointment. Most obvious, it violates the principle of civilian control over the military. This principle is no nicety; it’s essential both to democratic rule and to proper military strategy. Civilians set policy on war and peace; officers devise plans to execute it and even then under civilian supervision. The drafters of the 1947 law creating the Defense department took this principle so seriously that they barred officers from becoming secretary of defense until 10 years after they’d retired from the military. (This was later revised to seven years.) Congress has only once passed a waiver to this rule, in 1950, when President Harry Truman nominated retired five-star Gen. George Marshall to the post. (Marshall stayed on the job for a year.) Mattis is very popular on Capitol Hill and will almost certainly get a waiver as well.

However, as Erin Simpson, a scholar of civil-military relations, recently wrote, Marshall had been a staff officer, specifically the Army chief of staff during World War II. After that, and before taking over the Defense department, he served as Truman’s secretary of state. In other words, he’d dealt with national policy (e.g., the Marshall Plan) and bureaucratic rivalries. Mattis has experience at neither, and it’s not clear that he’d be good at either. And this is a man who has devoted his entire life to nothing but the history and conduct of war. He’s never been married and says he has no television. Broad as his perspective may be by military standards, it is that, narrowly, of a warrior.

Two points should be drawn from this. First, Mattis needs to appoint a very competent, professional, and preferably civilian deputy secretary of defense. The deputy is the person who runs the department’s day-to-day operations and oversees the white papers, the weapons procurement debates, and the rest of the grind and gruel of Pentagon politics. My guess is that Mattis will look at the issue of whether to buy more or fewer of certain weapons from the vantage of whether they help the fighting men and women in the field—and that’s good. But that instinct isn’t enough to run a gigantic bureaucracy with a $600 billion budget.

But more important than Mattis’ possible inadequacies as a manager are the questions about his suitability as a policymaker, and that’s crucial for this administration because Trump knows nothing about defense policy. He will, therefore, rely heavily on the secretary of defense for guidance—which means, in this case, he’ll be relying heavily on an all-too-recently-retired four-star general. Or, rather, he’ll be relying on retired Gen. Mattis when he’s not relying on his national security adviser, Lieut. Gen. Michael Flynn.

And here we’re whipped back to the view that Mattis might be a very good man to have not just at the helm of the Pentagon but sitting in the meetings of cabinet secretaries in the National Security Council, making sure that Flynn—who will run those meetings—doesn’t invoke his authority as a general to justify some harebrained scheme. Mattis can pull rank on this claim—not just as a retired four-star versus Flynn’s three but also as someone who’s read a lot about strategy, led military campaigns, and headed a regional combatant command (U.S. Central Command) versus an intelligence officer (granted, a very able one) whose strength was purely tactical (finding bad guys for special-ops fighters to go kill).

Trump ravaged the generals during the election campaign, said he knew more about ISIS than they did. But he seems to feel differently about his generals—and it may be no coincidence that the ones he’s picked so far were let go during the Obama years. (Flynn was fired for messing up his directorship of the Defense Intelligence Agency; Mattis was relieved from Central Command a bit earlier than scheduled for reasons that remain murky but seem to be related to his hawkish stance on Iran. Mattis doesn’t favor scrapping the Iran nuclear treaty, by the way, but doubts it will stop Iran from someday building a nuclear weapon.) That being the case, it’s a relief that Trump will have at hand the best of those let-go generals as well as one of the worst.

So, two cheers for Jim Mattis as a check on Trump’s pseudo–tough guy impulses and on Flynn’s crass prejudices.