President Trump, flanked by Secretary of State Rex Tillerson and Secretary of Defense Jim Mattis, speaks during a Cabinet meeting in December. (Jabin Botsford/The Washington Post)

The firing of Secretary of State Rex Tillerson is “a Washington, D.C., story” in most parts of the world, Defense Secretary Jim Mattis said Thursday, downplaying the ouster of an ally in the Trump administration in his first public remarks on the issue.

Mattis was in Afghanistan on Tuesday when the news broke that President Trump fired the nation’s top diplomat after not seeing eye to eye on several issues. A spokesman traveling with the Pentagon chief declined to discuss the news then, but Mattis did so on a military flight to Washington at the tail end of a five-day trip that also included stops in Oman and Bahrain.

Mattis, asked when and how he found out about the firing and what his reaction was, said that he “would prefer to not answer that.” But he suggested that the nature of American diplomacy will not change.

“The durability of our relations with these countries, with the United States having strong institutions, goes beyond personalities,” Mattis said, after spending the morning visiting with U.S. troops in Bahrain.

The defense secretary, who has often underscored the need for the United States to maintain relationships abroad, said his job is to ensure that U.S. ambassadors and other diplomats “always speak from a position of strength with support for international law.” Tillerson’s firing did not “even come up” on Mattis’s trip other than when reporters traveling with him asked about it, he said.

“I understand why you are asking,” Mattis told reporters. “But I’m just pointing out that in most parts of the world, this is a Washington, D.C., story. It’s not about whether the United States is still an ally to the countries I’ve been, or the countries I’ve been talking to. I’ve signed letters while I’ve been out here with the normal coordination. The job goes on.”

After months of tension, Trump signaled in a surprise tweet Tuesday morning in Washington that he was firing Tillerson and replacing him with CIA Director Mike Pompeo. Trump then called Tillerson a couple hours later from a flight on Air Force One to make sure that he was aware of the news, Tillerson said in a news conference Tuesday afternoon.

The turn of events marked an unceremonious exit for the secretary of state who was once the chief executive of ExxonMobil. He had grown unpopular with both the president and rank-and-file State Department employees who were disgusted with low morale in the department and how many jobs the administration has left unfilled.

The defense secretary seemed to find common ground with Tillerson over the first year of the administration, promoting diplomacy as the primary option to avoid conflicts and underscoring how ugly a war with North Korea would be. The two leaders met weekly for breakfast and discussed goals before meetings at the White House, Mattis said in December.

Tillerson, in his news conference Tuesday, thanked Mattis for his assistance and praised the work of both State Department employees and U.S. troops. He notably did not mention Trump.

Mattis, a former four-star Marine general, does not have the same long-term relationship with Pompeo that he does with some of the generals that Trump has appointed in his administration. But the two have collaborated over the past year while Pompeo was CIA director.

Like Trump, both men have taken a dim view of what they see as Iran’s destabilizing influence in the Middle East, although Mattis has said he thinks the United States needs to adhere to the nuclear deal that President Barack Obama and his administration signed with Iran. Trump has threatened repeatedly to pull out of the deal.

Mattis underscored his concerns about Iran’s actions again Thursday, telling reporters that the United States now has “worrisome evidence” that Iran is “mucking around” in Iraq’s election process and trying to sway candidates and votes ahead of their elections in May. He compared it to efforts that U.S. intelligence agencies have said Russia has made to influence elections in other countries.

The defense secretary, asked if Iraqi Prime Minister Haider al-Abadi is being undermined by the Iranian efforts, said he did not want to speculate about that. He did not detail what the evidence includes.

“But we know the money is going over and that they are doing what they can to impact the elections,” Mattis said. “We don’t like it when it happens in our country, and we certainly don’t want to see it in Iraq.”

U.S. Navy officials in Bahrain said on Thursday that Iran’s military has continued to ease up on harassing maneuvers it decried against Navy vessels, but detailed several ways they believe that Iran continues to destabilize the region. They include smuggling weapons into Yemen to back Houthi rebels in a civil war, testing unmanned boats packed with explosives and supporting the regime in Syria despite a slew of alleged war crimes the international community has documented.