The services of Defense Secretary Jim Mattis may soon no longer be needed, not because of any misstep, but because President Trump believes he’s outgrown him.

Trump has increasingly shown disdain for the military brass he once lauded as “his generals,” and has even mocked them in public and insisted he knows more than they do about dealing with allies and defending against enemies.

In a telling comment during his interview on the CBS news program "60 Minutes," Trump told Lesley Stahl that when it came to dealing with the NATO alliance, Mattis, a former supreme NATO commander, was essentially out of his depth.

“Frankly, I like Gen. Mattis. I think I know more about it than he does,” Trump said, “And I know more about it from the standpoint of fairness, that I can tell you.”

At a rally in Iowa this month, Trump regaled the crowd with a story about how he shut down “a certain general” who he described as a “good guy, but, you know, he's not into business. He's into winning wars.”

The anecdote was an apparent reference to former national security adviser H.R. McMaster, another general who fell out of favor with Trump over time.

When Trump picked Mattis as his defense secretary at the start of his administration, Mattis was praised as “the saucer that cools the coffee,” as Democrat Jack Reed put it at Mattis’ January 2017 Senate confirmation hearing.

By all accounts, Mattis has lived up to expectations and served a moderating force on some of Trump’s more impulsive instincts. He convinced Trump not to pull U.S. troops out of Afghanistan and not to launch a decapitation strike against Syria’s Bashar Assad, and has reassured nervous allies after Trump gives them a tongue-lashing over Twitter.

Now, many worry that Mattis is the last man standing between Trump and a more muscular foreign policy that could lead to war.

“The risk is that [national security adviser] John Bolton and Secretary of State Mike Pompeo, left ‘home alone,’ will encourage Trump’s more reckless instincts. The danger grows that a president beset with domestic political problems will hurl the nation into an international crisis to try to rally support,” argued Joe Cirincione of the arms control advocacy group the Ploughshares Fund.

“Without Mattis, Bolton and Pompeo’s pressure campaign on Iran will certainly intensify and could explode into a new, disastrous war," Cirincione said.

Despite that risk, some say Mattis' departure seems increasingly imminent, as Mattis is simply out of step with Trump’s "America First" policy, which is designed to disrupt the old ways of doing things.

“The Mattis worldview reflects the beliefs of the Washington policy establishment. He defends a global status quo that the president is determined to transform,” said Loren Thompson of the Lexington Institute.

“When the president says Mattis is 'sort of a Democrat,' what he means is that the views the defense secretary espouses helped create the security challenges the administration inherited — starting with China and North Korea,” Thompson said.

There are already signs Mattis has less standing with Trump than he once did. As Trump has grown more comfortable and confident as president, he’s has less use for the conventions of foreign policy, and as a result, Mattis has been relegated to the B-team, busying himself with using the new infusion of fresh funding to rebuild the parts of the military that were slowly sliding into decrepitude.

Mattis’ special talent seemed to be gently steering the president away from his initial shoot-from-the-hip response to more measured options, but he’s lost as many battles as he’s won. He failed to dissuade Trump from withdrawing from the Iran nuclear agreement, which he saw as flawed but better than nothing. He lost the argument about whether to create a separate Space Force, which the Pentagon was convinced would lead to needless overhead and duplication.

If Mattis is shown the door, the big question would be who would replace him.

“I would worry about losing Mattis’ ability to calmly reason with the president about how to handle crises, without directly confronting or angering him, yet ultimately helping him slow down and see things in a more balanced light,” said Michael O’Hanlon of the Brookings Institution.

But Todd Harrison of the Center for Strategic and International Studies says depending who replaces Mattis, it's possible there would be little change at the Pentagon.

“The focus of the department in the National Defense Strategy, something Mattis has really championed, is focusing more on Russia and China, less so on counterterrorism and insurgencies, but that shift in focus predated Mattis," Harrison said.

In the "60 Minutes" interview, Trump admitted it took him awhile to get his “sea legs,” but now he’s very comfortable being the commander in chief.

“Now I very much feel like POTUS. I do. I feel like the president,” he said.

As for Mattis’ future, Trump said, “He may leave. I mean, at some point, everybody leaves. Everybody. People leave. That's Washington.”

Mattis, who is a student of the Constitution, is well aware he serves at the pleasure of the president, and as student of history, he is no doubt also aware of the famous observation variously attributed to Georges Clemenceau or Charles de Gaulle that “cemeteries are full of indispensable men.”