James Mattis was left out of the loop on issues such as Iran, Korea and a new space force but his departure would alarm allies

James Mattis has lost influence in the Trump administration, and his future as US defence secretary is now in question, according to former administration officials and analysts.

His eclipse is a worrying prospect for US allies, whom Mattis has worked hard to reassure by mitigating Donald Trump’s unilateral actions and disparaging remarks with promises of sustained solidarity and cooperation.

In the nervous run-up to July’s Nato summit, European partners in particular view the defence secretary as a talisman of continuity in a capricious US administration.

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However, there are signs that he is becoming increasingly adrift from the White House. According to an NBC News report, Mattis was left out of the loop on a number of major Trump decisions, such as the abrogation of the Iran nuclear deal in May, the abrupt suspension of joint military exercises with South Korea and the presidential executive order creating a new “space force”, against Pentagon advice.

For most of his time as defence secretary, Mattis has sought to walk a verbal tightrope, being as supportive as possible to the commander-in-chief while avoiding outright lies. He has managed this to some extent by keeping a low profile. As the months have passed, however, he has found it harder to remain factual and loyal at the same time.

After the Singapore summit with Kim Jong-un, Trump and Mike Pompeo sought to give the impression that the joint statement by the two leaders was a diplomatic triumph that had begun the process of North Korean disarmament.

Asked if he could put his finger on any evidence of disarmament by Pyongyang, Mattis admitted he was “not aware” of any, flatly contradicting Trump. He added: “The detailed negotiations have not begun.”

Mattis has been an exception in Trump’s cabinet, in that his refusal to flatter or swear personal loyalty to the president was tolerated. Trump admires his tough-guy reputation and his bond with the men and women in uniform.

Facebook Twitter Pinterest Donald Trump talks to James Mattis at his inauguration parade. They talk less these days. Photograph: Erik S Lesser/EPA

Though the two men are reported to talk much less often than in the early months of the administration, there are no reports of the president denigrating his defence secretary to friends and acquaintances.

That was the humiliating fate of former secretary of state Rex Tillerson and former national security adviser HR McMaster. The widespread support the former marine general commands across the political spectrum has given Mattis greater staying power in office.

“With Mattis, the blowback would be greater than with anyone Trump has previously let go,” said Michael Carpenter, a deputy assistant secretary of defence in the Obama administration. “Mattis is highly respected in the Republican establishment.

“Unlike Tillerson who really screwed up management at the state department, Mattis is a competent manager,” said Carpenter, now a senior director of the Penn Biden Center for diplomacy and global engagement.

“Mattis would be the hardest person to replace. He is the most protected of any member of the cabinet,” Thomas Ricks, a military historian and the author of books on US military command and the marine corps, said. “But the iceberg is beginning to melt a little. There is a new assertiveness in Trump. He was scared of the presidency in the first six months, but he has gone to being the old Trump.”

Ricks said that regression involved increasing distrust of advisers in favour of Trump’s own gut instincts. Julianne Smith, former deputy national security adviser to Vice-President Joe Biden, said Mattis could be driven to resignation.

“He feels a sense of duty and he is committed to the job, and he has done a fabulous job in restraining some of Trump’s worst instincts,” said Smith, now director of the transatlantic security program at the Centre for a New American Security thinktank in Washington.

“If he gets into the position that he is not being consulted and he is being asked to do things he fundamentally does not agree with … he has to ask himself: do I exit stage right now or hang on to the bitter end, knowing that’s embarrassing and usually doesn’t end well.”

Facebook Twitter Pinterest James Mattis reviews a Chinese honour guard during a welcome ceremony at the Bayi Building in Beijing on Wednesday. Photograph: Pool/Reuters

The view that Mattis’s position is becoming weaker is not universally shared. Evelyn Farkas, a former deputy assistant secretary of defence for Russia, Ukraine and Eurasia, said: “I have seen no evidence that Trump’s respect for Mattis has been eroded or shaken.

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“I’m not sure that anything has really changed, except that more time has gone by, and more of these cases [of Trump making defence policy without consulting Mattis] have accumulated,” Farkas said, adding that the departure of allies Tillerson and McMaster has made it harder for Mattis to prevail in policy arguments.

Much could happen in the coming months to change the calculations of both the president and his defence secretary. If Trump makes the same kind of impromptu concessions when he meets Vladimir Putin in mid-July, as he did to Kim in Singapore, it will become much harder for traditional security policy conservatives like Mattis to stay on.