President-elect Donald J. Trump has chosen retired Marine Gen. James Mattis to be his secretary of defense. His decision percolated out late Thursday afternoon through anonymous sources close to the transition, who spoke with the Washington Post, and will likely be formally announced next week.

Last week, Trump called Mattis “the real deal.” He told the New York Times that while Mattis shares his love of “winning,” the two men disagree about waterboarding and other forms of torture. Mattis, Trump said, has never found torture to be useful. His preferred tools for getting answers are “a pack of cigarettes and a couple of beers.”

It isn’t clear whether Mattis, the former head of U.S. Central Command, actually succeeded in changing Trump’s mind regarding torture. Trump insisted that he hadn’t, and that if the American people wanted more torture, he would deliver it. But Mattis did manage to draw a line between his own thinking and that of his prospective boss, far more so than Gen. Michael Flynn, Steve Bannon, and others who have managed to hitch their sputtering careers to Trump’s butterscotch locomotive.

“Gen. Mattis made a practical argument, not a moral argument,” says retired Col. Mark Cancian, a senior advisor at the Center for Strategic and International Studies. “That proved to be convincing.”

It isn’t easy to convince Trump of anything. His flirtation with a Mattis appointment suggests that he could be developing a capacity to hear voices that are not echoes of his own.

Mattis is exactly what Trump is not, a soldier-scholar who knows something of the wider world. Now 66 years old, Mattis was born in Walla Walla, Washington. His lifelong bachelordom is the source of one of his many nicknames: “warrior-monk.” He served in every major U.S. Middle Eastern conflict from the first Iraq War on. In 2001, as a one-star general, he led 4,000 Marines in a search for Osama bin Laden near the Afghanistan/Pakistan border. In 2004, as a two-star, he led a Marine division into the second battle for Fallujah. He went on to lead combatant commands at the Pentagon and NATO, culminating in two years as the head of Central Command under President Barack Obama, reportedly leaving after disagreeing with Obama’s policy on Iran. Shortly before his departure, Mattis appears to have weighed in with the Pentagon on behalf of Theranos, Elizabeth Holmes’s troubled biotech firm. He later joined the company’s board. Should Trump nominate Mattis, emails between Mattis and Holmes are likely to come up during his Senate confirmation hearing.

Trump actively avoided military service. Mattis, who reportedly has a library of six thousand books, could compensate for this lack of familiarity with the military, as well as the president-elect’s more general forms of ignorance. The fruit of Mattis’s wide reading is apparent from this 2003 email to a colleague:

The problem with being too busy to read is that you learn by experience (or by your men’s experience), i.e. the hard way. By reading, you learn through others’ experiences, generally a better way to do business, especially in our line of work where the consequences of incompetence are so final for young men. … We have been fighting on this planet for 5000 years and we should take advantage of their experience.

Professor Richard Kohn, a military historian at the University of North Carolina, called Mattis “loyal and discreet. He doesn’t talk out of school. He seeks out top people, and people like working for him.” Trump, Kohn continued, “is going to be advised by a National Security Advisor [Flynn] with some deep flaws. I think that having a legendary and respected retired general in charge of Defense makes a great deal of sense.”

“I think he would be an outstanding candidate,” Michèle Flournoy, widely believed to be Hillary Clinton’s frontrunner for secretary of defense, told NPR. In 2010, Seth Moulton, a decorated Marine captain who served in Iraq, praised Mattis as one of the “leaders who can speak the truth, who aren’t just constrained by the politics of the moment.” Moulton is now a Democratic congressman representing the Sixth District of Massachusetts.

Mattis has also gotten cheers from veterans and Trump supporters online, in the form of celebratory memes dubbing him the Patron Saint of Chaos (Chaos was Mattis’s call-sign in Iraq and Afghanistan), praising his lethal “double knife hands,” and saying that he “Puts the Laughter in Manslaughter.”

Indeed, Mattis is famous for speaking bluntly when it comes to describing the military’s primary function — killing the enemy — and for whistling about his work. He has lived down an eleven-year-old gaffe where he described killing as “a lot of fun, … a hell of a hoot,” but has never backed off from the stance that the military’s core function is not peacekeeping or humanitarian missions but war-fighting. He is widely credited with popularizing the motto of the 1st Marine Division — “No Better Friend, No Worse Enemy” — and turning it into a basic tenet of counterinsurgency doctrine. His letter to the division on the eve of the 2003 invasion is a cool-headed exhortation to “close with those forces that choose to fight, and destroy them.”

William Treseder, a former Marine who now advises startups, said that Mattis is skilled at injecting a fighting spirit into mundane jobs. “The enemy should quiver in fear every time you sign a contract,” was Mattis’s advice to a fellow soldier working in procurement, Treseder said

While Mattis briefly flirted with his own 2016 presidential run, he chose not to leap into politics with the gusto of Flynn or retired Gen. John Allen, both of whom delivered fire-breathing speeches at this year’s major-party conventions. Adm. Mike Mullen, the former chair of the Joint Chiefs of Staff, criticized both Flynn and Allen at a panel discussion last week for injecting politics into the military.

“Too many times when you see retired military individuals … take a very strong political stance, that sends the wrong message to America,” Mullen said. “It sends the wrong message back inside our military. Because it teaches our young ones that it’s okay. And it’s not okay; … it’s a fundamental principle of the United States of America that the military has got to stay apolitical.”

Keeping the military out of politics and under civilian control is one reason that the 1947 National Security Act requires that officers be out of military service for ten years before assuming the mantle of secretary of defense. In 2008, Congress lowered the waiting period to seven years. Congress granted a waiver to Gen. George Marshall, President Truman’s third secretary of defense, in 1950. Mattis would need his own congressional waiver to serve under Trump.

If Mattis and Trump still have disagreements to smooth over, the treatment of Gold Star families, whose children have died fighting for the U.S., would likely be one of them. During the campaign, Trump suffered a blistering attack from the family of Army Capt. Khizr Khan. He chose to push back, suggesting that Khan’s mother was not allowed to speak at the Democratic National Convention. He also appeared to draw an equivalence between his own struggles in business and the sacrifice made by the Khan family.

“I’ve made a lot of sacrifices,” Trump said in an interview with ABC News. “I work very, very hard.”

Trump’s words may have served to aggravate a trend that Mattis pointed out in an anthology he recently co-edited about the military-civilian relationship in the U.S., which describes the “atrophying” of empathy for Gold Star families. “What had been a more common experience of loss in previous wars now tends to be an isolating experience for families,” Mattis wrote, with his co-editor.

Should Mattis join the cabinet of such an unusual commander-in-chief, one of his challenges will be to balance the roles of servant and tutor.

Update: Nov. 30, 2016
This story has been updated to reflect reports that President-elect Trump has selected Mattis as secretary of defense, and an official announcement is expected soon.