General James N. Mattis [United States Department of Defense]

Donald Trump’s pick for Secretary of Defense is an outspoken former Marine Corps general known as “Mad Dog,” but don’t let the name fool you. James Nicholas Mattis is an insightful, charismatic leader whose motivational abilities and battlefield successes came with nuanced understanding of history, humanity and his enemy.

Also called “the Warrior Monk,” the well-read native of Pullman, Wash., joined the Marines at 19 and retired three years ago after heading Central Command, where he oversaw wars in Iraq and Afghanistan and conflicts in Syria and Yemen.

There were signs even before early December’s announcement that the 66-year-old, who’s never owned a television, never been married and has no children, was influencing the president-elect on key policy issues.

Trump told The New York Times that he had asked Mattis his views on the practice of waterboarding, a form of torture widely used by U.S. intelligence post-9/11 but since abandoned. Trump pledged during the raucous presidential campaign that he would bring it back, much to the delight of his core supporters.

Trump told the newspaper that the general’s answer surprised and impressed him. “He said: ‘I’ve never found it to be useful.’ He said: ‘I’ve always found, give me a pack of cigarettes and a couple of beers and I do better with that than I do with torture.’”

Mattis, the same man who famously said “be polite, be professional, but have a plan to kill everybody you meet,” also warned his troops: “If in order to kill the enemy you have to kill an innocent, don’t take the shot. Don’t create more enemies than you take out by some immoral act.”

Before splitting with Obama on Iran policy, he co-authored a counterinsurgency manual aimed at limiting sectarian violence in Iraq. “Whenever you show anger or disgust toward civilians,” he said, “it’s a victory for al-Qaida and other insurgents.”

After Trump threatened to pull the plug on American defence pacts like NATO, calling allies who didn’t pay their share “freeloaders,” Mattis responded last spring: “For a sitting U.S. president to see our allies as freeloaders is nuts.”

Even on Iran, Mattis is seen by many as a voice of reason and potential moderating influence among the hawks advising Trump. He has questioned the benefits of the Iran nuclear deal but said “there’s no going back” on it.

Mattis’s appointment was confirmed in the U.S. Senate on Jan. 20.