Susan Walsh / AP

James Mattis was an unusual choice for secretary of defense, an explicitly civilian role. It wasn’t so long ago that Mattis was the commander of Centcom, America’s sprawling military presence in the Middle East and Central Asia. Sure enough, there was a widespread sense in the Pentagon that Mattis was more inclined to trust senior military officials, many of whom he came to know over the course of his decades-long career as a Marine officer, than the senior civilian officials charged with overseeing uniformed personnel.

Given the importance of civilian control over the military, one might expect Mattis’s unorthodox approach to leading the Pentagon to have raised concerns among leading members of the national-security community. But it did not, for the simple reason that in contrast to President Donald Trump, whom he served only with great ambivalence, Mattis firmly adhered to the bipartisan internationalist consensus on foreign and defense policy. It is therefore not surprising that his departure has set off a panicked reaction.

Particularly at the start of the Trump presidency, Mattis was a reassuring figure. He was living proof that despite Trump’s seemingly endless series of provocations, U.S.-led alliances would remain intact, and indeed might be strengthened. Consider that though the president has loudly and persistently questioned the wisdom of the U.S. commitment to the defense of its European allies, U.S. military spending in Europe has increased from $789 million to $4.77 billion during his tenure, with more expected to come. This was very much Mattis’s handiwork, as was the Pentagon’s new National Defense Strategy, which called for a greater emphasis on great-power competition and meeting the challenge posed by a revisionist China.

For some time now, however, it has been clear that Mattis’s influence has been waning, as Tobin Harshaw observed late last month. Having failed to persuade Trump to remain in either the Iran nuclear deal or the Paris climate agreement, Mattis attempted to play the role of good soldier when it came to various other presidential priorities, with evident strain. Once the president decided that he would no longer tolerate an ongoing U.S. military presence in Syria, despite his defense secretary’s firm objections, Mattis’s departure was a foregone conclusion. Rather than remain in office a tortured man, mouthing support for the president’s agenda by day while working tirelessly to stymie it by night, Mattis has chosen to clear the way for a successor who is more aligned with Trump’s sensibilities.

And that is as it should be. As president, Trump deserves to have a secretary of defense who agrees with him that the U.S. military should withdraw from its foreign military adventures, and who has the bureaucratic skill to help him implement that policy in a responsible way—not, as we’ve just seen in the case of Syria, by repeatedly trying to make it happen, failing, and then peremptorily ordering it done. Though the precipitousness of Trump’s planned Syria withdrawal has been roundly condemned, there are many serious scholars who accept that, as the political scientist Daniel Byman recently observed, “the logic behind the continued deployment of the 2,000 or so U.S. troops in Syria was always a bit fuzzy.”

Others, taking a broader view, maintain that the greater Middle East is fast becoming a strategic backwater, and that the United States would do well to focus its diplomatic and military resources on the Indo-Pacific. A defense secretary who accepted Trump’s insistence that the Syria mission not be open-ended might very well have persuaded the president to accept a more deliberate process. That is exactly the defense secretary Trump needs.

As for who could fit the bill, that is a harder question to answer. Thus far, speculation has centered on a handful of retired Republican eminences and one or two of the president’s allies in Congress, but it isn’t clear that any of them have the requisite familiarity with the workings of the Pentagon. Trump could reach out to a veteran of the Obama or Clinton administrations who shares his anti-interventionist instincts, but it is unlikely that such an overture would be welcomed. Don’t be surprised if the president winds up tapping another recently retired general for the job—one for whom a sense of duty is reason enough to endure the frustrations it will no doubt entail.

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.