In the turmoil of the Trump administration’s first year, Defense Secretary Jim Mattis could always count on the steady judgment of Rex Tillerson to help him present a united front to the president.

Whether about doubling down on Afghanistan, or opposing tariffs on U.S. allies, or confronting Russia, Tillerson was always a kindred spirit.

Though coming from vastly different backgrounds, Mattis with his distinguished four-decade military career, and Tillerson as a globetrotting energy executive, the two were seen as an effective duo and made up two-thirds of the so-called "suicide pact" with Treasury Secretary Steven Mnuchin. The three reportedly have agreed to leave if one of them is fired.

And as a strong believer in diplomacy to resolve conflict, Mattis, a grizzled former Marine commander, made no secret of his close relationship with the America’s top diplomat.

“It starts with me having breakfast every week with Secretary of State Tillerson,” Mattis said in January as he was rolling out the National Defense Strategy. “We talk two, three times a day, sometimes. We settle all of our issues between he and I, and then we walk together into the White House meetings. That way, State and Defense are together.”

Mattis was in Afghanistan when Trump made the announcement that he was firing Tillerson. Tillerson returned from Africa Tuesday morning.

“I thought the team was functioning well and had balance,” said James Carafano of the Heritage Foundation. “They put out a good series of solid foreign and security policies. It remains to be seen how this affects the ability of Trump’s team to storm and norm.”

Monday was just another day in which Tillerson seemed to be on a different page than the White House.

While press secretary Sarah Sanders avoided mentioning Russia, Tillerson issued a full-throated endorsement of the British assessment that Russia was likely responsible for last week’s nerve agent attack, which he called “the attempted murder of a private citizen on the soil of a sovereign nation.”

Trump admitted Tuesday that he left Tillerson out of the loop when he decided to accept Kim Jong Un’s invitation for face-to-face talks about North Korea’s nuclear program. He also said flatly that one reason for Tillerson’s ouster was a fundamental disagreement over the Iran nuclear deal, which Trump has called "worst deal ever negotiated."

Mattis always deferred to Tillerson on matters of diplomacy, a habit he will likely continue as Mike Pompeo, who Trump plans to nominate for secretary of state, takes over.

“This is a diplomatically led effort,” Mattis told reporters traveling with him this week. “So I do not want to talk about Korea at all. I'll leave it to those who are leading the effort, the State Department and the NSC.”

Mattis, too, has supported staying in the deal with all of its imperfections, while Pompeo has views more in line with the president's.

“There is no reason that Pompeo's appointment should present problems for Secretary Mattis, or in relations between State and Defense,” said Anthony Cordesman of the Center for Strategic and International Studies. “If anything, Pompeo has a military background and has focused more of his recent time on national security issues.”

Few doubt that Mattis will adapt and develop a similar working relationship with Pompeo.

“Mattis has proved to be a skilled bureaucratic player and will continue to play the dominant role in shaping national security strategy,” said Mark Cancian, a senior adviser with the Center for Strategic and International Studies.

“Mattis and Tillerson have agreed on many key points about national security, particularly on the importance of allies and the use of force. Pompeo's views here are less clear" Cancian said.

“Mike Pompeo will be a better teammate for both the president and Mattis,” said Loren Thompson of the Lexington Institute. “He shares the president's sensibilities, he served in the military, and he is a first-rate intellect. He also has a far better grasp of how Congress operates.”

Harry Kazianis, with the Center for the National Interest, said the firing was a long time coming.

“How can anyone actually be shocked that Donald Trump fired Rex Tillerson — and on Twitter no less? This has been rumored for months and I would wager has been in the offing for weeks," he said.

“The simple fact is President Trump and now former Secretary of State Rex Tillerson never had the personal chemistry needed to make this marriage work,” Kazianis said. “And I can predict what happens next: [national security adviser] H.R. McMaster will also soon get the axe. All for the same basic reason — Trump and McMaster are also not a good fit.”