The 67-year-old four-star Marine general had forged a front with Tillerson to curb Trump's most extreme tendencies, like making a sudden break with traditional allies or threatening nuclear war with North Korea.

It was a quiet alliance, helped by the fact that Mattis's Washington residence was near the State Department, giving the two a chance to talk regularly over breakfast.

"I inform Secretary Tillerson of the military factors. And we make certain that then, when we come out of our meetings, the State Department and Defense Department are tied tightly together, and we can give straightforward advice to the commander-in-chief," Mattis told CBS last May.

Tillerson paid homage to their partnership in a short exit speech on Tuesday, after the president dismissed him in humiliating fashion -- by tweet.

"I am told for the first time in most people's memory the Department of State and the Department of Defense have a close working relationship, where we all agree that US leadership starts with diplomacy," Tillerson said.

But it is Mattis, the battle-seasoned Marine, who has proven to have the deft diplomatic skills needed to work with the mercurial Trump.

While Tillerson and White House national security advisor HR McMaster have frequently become public targets of Trump's anger, the Pentagon chief remains in good stead, even if he frequently disagrees with the president.

He has proven able to coax his boss away from inflammatory rhetoric and towards more traditional US positions.

- Diplomacy first -

For instance, when Pyongyang last year blasted a ballistic missile over Japan, Trump warned that military options were on the table and insisted negotiations were "not the answer."

Mattis, together with Tillerson, quickly dialed the message back, saying the United States was never out of diplomatic options. And he has continued to insist America's response to the crisis should be led by the State Department.

On Pentagon affairs, Mattis can also quietly hold the president at bay.

Last year, Trump said he would ban all transgender personnel from serving in the military. But the move has yet to be implemented at the Pentagon, pending legal challenges.

To avoid friction, Mattis has studiously avoided tons of media coverage -- as Trump has a clear dislike for members of his team who pull attention away from him.

Rather than get in front of TV cameras, Mattis speaks to journalists on his own terms, showing up informally in their area of the Pentagon when he has something to say.

And he avoids making any comment on the president, even in private, always saying that he is not a part of political decision-making. He rails against any suggestion of fundamental disagreements between them.

"If I say six and the president says half a dozen, they're going to say I disagree with him," Mattis tells reporters. "So let's just get over that."

Indeed, he has stayed absolutely silent on the firing of his closest cabinet ally -- helped by the fact that it took place while he was on a trip to Oman and Afghanistan.

Mattis's spokesman Jeff Davis declined to comment on Tillerson's dismissal.

- Daring to disagree -

Mattis does at times make his differences with the president known.

He added his name recently to a letter to Commerce Secretary Wilbur Ross opposing Trump's proposed sharp hike in taxes on steel imports, saying it would damage national security.

When Trump pushed ahead on the tariffs, Mattis opted for silence.

And he and his officers continue to challenge the president on the value of the nuclear accord with Iran. Trump has called to scrap it, but Mattis has repeatedly pushed back, saying it is in the US national interest.

That raises questions about how Mattis will relate to Tillerson's replacement, Mike Pompeo, who for the past year has served as director of the Central Intelligence Agency.

At least publicly, Pompeo's views have aligned more with those of Trump, making him one of the president's favorites in the national security realm.

But that means Pompeo is more opposed to the Iran deal, and has been more aggressive on North Korea, where Mattis and Tillerson favored diplomacy rather than military conflict.