Mattis is the last of the men Trump once called “my generals” to leave the administration. But there are generals and there are generals, and then there’s Mattis. Marines I know who served under his command in the initial phases of the Afghanistan and Iraq wars speak of him with respect bordering on reverence. His mastery of speed and aggression in combat—his “Mad Dog” qualities — are the least of the reasons why.

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I think their esteem is rooted in the extreme seriousness with which Mattis approaches every task. I don’t mean humorlessness; the general has said that “a sense of humor is like body armor around your body. It’s armor around your spirit and it keeps your spirit from going grim.”

What I mean is that Mattis believes that his work is deeply important. It is his responsibility to be prepared and to be knowledgeable, to plan thoroughly, to foresee pitfalls and consequences, to build a harmonious team and to share his vision and aims with every team member with such clarity that all are able to act decisively on their own to further the mission. Any mistakes or omissions on his part as a leader can result in the unnecessary deaths of his Marines, and he conducts himself accordingly.

Never married, Mattis has a monkish devotion to his calling. He reads voraciously, especially in the areas of military history and the art of leadership. His strategy for unseating the Taliban in Afghanistan, perhaps the most audacious amphibious operation in Marine Corps history, was informed by his knowledge of British operations in World War II Burma and the fine details of Ulysses S. Grant’s Vicksburg campaign. For the invasion of Iraq, he reviewed the career of Alexander the Great. He applied the dialetical method of philosopher G.W.F. Hegel to better see through the fog of war. “By reading, you learn through others’ experiences — generally a better way to do business,” Mattis has observed, “especially in our line of work where the consequences of incompetence are so final for young men.”

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Likewise, Mattis cultivates the asceticism of a monk, eschewing the trappings of a large retinue in favor of the leanest possible command staff. When Mattis learned that he could free up the equivalent of eight medium-size trucks for other uses in Iraq simply by leaving cots behind, he ordered every Marine to sleep on the ground — officers as well as enlisted men.

This picture of the departing secretary is painted in “The Mattis Way of War” by Michael L. Valenti, a Marine Corps major assigned to the Command and General Staff College at Fort Leavenworth, Kan., in 2014. Reading it, I began to see that Mattis was never the sober complement to the mercurial president. He was Trump’s opposite, for whatever else you might say about the president, he is not a man who takes his job seriously. He has not prepared for it studiously; he hasn’t steeped himself in its history and nuances; he doesn’t test and retest his assumptions, nor does he minimize risk.

Mattis’s letter of resignation was a model of the clear expression that Valenti identified as a hallmark of his leadership. He explained that his deep experience of the world has led him to value America’s friends, to know and oppose America’s enemies, and to see clearly which are which. A believer in the importance of “harmony” in groups under pressure, the general concluded that his hard-won insights have no place on Trump’s unserious team.

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In the sad aftermath of this honorable man’s realization that American government has no place for the likes of him right now, I was struck by this in Valenti’s paper. Mattis says “the greatest threat to the Marine Corps is not the enemy, but ‘a leader who is not admired.’ ” Admiration is the foundation of trust; trust the lubricant of efficiency and effectiveness. When an admired leader sets a clear and well-prepared course, the team can move swiftly to achieve it.

When Mattis visited the Oval Office to tender his resignation, the room contained an admirable leader. The threat and sorrow is that this was the one who walked away.

Read more from David Von Drehle’s archive.