Donald Trump has chosen a man to run the Pentagon who is in important ways the president-elect’s opposite, and many experts think that’s a good thing.

James Mattis went by the call sign “Chaos” and was widely known as “Mad Dog” and “the Warrior Monk” when he was a Marine Corps general. He earned a reputation as a blustery cowboy. “It’s fun to shoot some people,” he once said, one of many such Patton-like quotes.

But those who know Mattis best say he is thoughtful, well-read, seasoned, articulate and easy-going — in other words, quite different from Trump.

Joe Plenzler, a retired Marine Corps lieutenant colonel who was Mattis’ spokesman for several years and served about a dozen other generals, calls Mattis “the best of the best.”

“He is the finest military mind our nation has produced in a long, long time,” Plenzler said in an interview.

[Mattis Could Clash in Confirmation Over PTSD, Women in Combat]

The differences between Trump and Mattis go beyond style. Mattis has stated positions on national security issues that contrast with Trump’s, to the degree the president-elect has solid positions at all.

Mattis has already caused Trump to reconsider his support for using waterboarding and “a hell of a lot worse” on alleged terrorists in detention.

In a meeting last month, Mattis told Trump extreme interrogation methods don’t work as well as “a pack of cigarettes and a couple of beers” in getting detainees to talk, according to Trump’s account a few days later during a meeting with The New York Times.

“I’m not saying it changed my mind,” Trump said of Mattis’ view on torture. “Look, we have people that are chopping off heads and drowning people in steel cages and we’re not allowed to waterboard. But I’ll tell you what, I was impressed by that answer.”

Will Mattis just impress Trump going forward? Or will he change his mind? That is the question.

Take Russia. Whereas Trump has declined to criticize Russian President Vladimir Putin and has, on many occasions, praised him, Mattis has been a leader in the U.S. military’s chorus of concern about Putin. For instance, Mattis told The Heritage Foundation last May that Russia is trying to break NATO apart and is a “severe” and “serious” threat — more so than most know, the retired general said.

Or consider Iran. Trump has called the nuclear agreement with Iran a “disaster” and “the worst deal ever.” Mattis is indisputably a hawk on Iran, calling it America’s top threat in the Middle East.

But Mattis was clear in a speech to the Center for Strategic and International Studies in April that he does not want to scrap the deal.

“One point I want to make … is there’s no going back,” Mattis said.

[Opinion: Don’t Break the Iran Nuclear Deal]

Trump has threatened to rescind U.S. security guarantees for any NATO partner nation that does not spend enough on its defense, and he has criticized allies for not doing enough to bear the financial burden of their own defense. Mattis, by contrast, has been an advocate of alliances and diplomacy. He once served in NATO’s upper echelons.

Mattis recently told the Senate Armed Services Committee, “If you don’t fully fund the State Department, then I need to buy more ammunition.” He criticized President Barack Obama for bemoaning in a published interview America’s so-called free-rider allies.

“He has long been a forward-looking strategic thinker, championing the use of America’s civilian tools alongside the military,” Liz Schrayer, president of the U.S. Global Leadership Coalition, said in a statement. Schrayer’s group advocates the importance of development and diplomacy alongside defense.

One of the biggest defense issues Trump and Mattis, if confirmed, will have to grapple with is fitting nearly $1 trillion in nuclear weapons modernization spending into the budget. Trump’s views on nuclear matters, again, to the degree they exist, are not well-known. During one of the GOP presidential debates, he said, “Nuclear is just the power, the devastation is very important to me.”

Mattis has already shown he is ready to take a hard look at previously sacrosanct elements of the nuclear arsenal.

“Is it time to reduce the triad to a dyad, removing the land‐based missiles?” Mattis asked at a Senate Armed Services hearing last year. It was a question, not a statement, but most Republican hawks are not asking the question, at least aloud.

Mattis was also highly critical of Trump’s since-modified proposal to ban all Muslims from entering the United States.

“This kind of thing is causing us great damage right now, and it’s sending shock waves through this international system,” Mattis said.

Jon Soltz, chairman of VoteVets.org, a progressive group, suggested in a statement that Trump and Mattis are likely to clash on the issues.

[Kaine, Other Senators Take Sides in Clash Over Nuclear Arms]

“As someone who has stood diametrically opposed to Donald Trump on issues like Iran and NATO, will he stand up to Trump and build a coalition of voices in the administration to challenge Trump when he is wrong?” Soltz asked of Mattis. “In a great many cases, the last bulwark against Donald Trump making catastrophic decisions will be General Mattis.”

Ellen Tauscher, an undersecretary of State for arms control early in the Obama administration, said Friday she hopes the result of those debates will be more critical thinking, thanks to Mattis, on issues like nuclear weapons, the Iran nuclear deal and Russia relations.

Tauscher, a former senior Democrat on the House Armed Services Committee, called Mattis an innovative thinker and problem solver who is well-read and scholarly. The retired four-star general can talk tough and be blunt, but he’s “not somebody who shoots from the hip.”

Not all observers are sanguine about Mattis’ prospects for success in the Pentagon. Gordon Adams, who was in charge of defense programs in President Bill Clinton’s Office of Management and Budget, said in an interview that generals used to getting results are typically frustrated by the cumbersome politics in Washington.

“The history of muddy-boots generals doing well in the political morass in Washington is not very great,” Adams said. “Washington requires stealth, guile, compromise, negotiation and moving problems to the right. … Nine times out of 10, any general I’ve ever worked with is frustrated by Washington.”

But Plenzler, Mattis’s former spokesman, said Mattis has one skill that will serve him well no matter where he is. “He’s a student of people,” Plenzler said.

Megan Scully contributed to this report.