Few Americans have better served their country than Jim Mattis, a legendary Marine general and — until last December — Secretary of Defense, who probably saved America from some of President Donald Trump's worst impulses.

He's as tough as his reputation and also a scholarly story teller. His recently released memoir, "Call Sign Chaos," is interesting, instructive and, ultimately, unsatisfying.

Mattis offers candid criticisms of former presidents George W. Bush and Barack Obama Barack Hussein ObamaDemocrats ramp up pressure on Lieberman to drop out of Georgia Senate race The Hill's Campaign Report: Biden on Trump: 'He'll leave' l GOP laywers brush off Trump's election remarks l Obama's endorsements Trump pledges to make Juneteenth a federal holiday, designate KKK a terrorist group in pitch to Black voters MORE and ex-Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE; he lays out what are clear policy differences with the current administration, but barely mentions Trump and totally ducks his fitness for office.

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And this bright, exceptionally well-read American hero does this under the false pretense that it's a tradition that former defense secretaries don't criticize the commander in chief — and neither do retired military generals, especially Marines.

Except that's not so.

During the Clinton administration, former Republican defense chiefs Donald Rumsfeld and Dick Cheney were both openly critical. Cheney — a candidate for vice president at the time — assailed the national security policies as leading to a U.S. military "in decline."

More recently, Bill Cohen, Defense Secretary in the late 90s, charged Trump with taking "a wrecking ball" to every pillar of global security, proving he's unfit to be commander in chief.

At points, Mattis suggests it's different with a current president. Yet both Bob Gates and Leon Panetta, two of Obama's defense secretaries, wrote books with some direct criticism of the 44th president while he was still in office. (The timing is open to criticism, though neither questioned Obama's basic fitness to be President.)

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That's where Mattis falters. Journalist Bob Woodward, in a book last year, reported with specificity that Secretary Mattis was so outraged at Trump's shallowness on North Korea that he said the president has "the understanding of a fifth or sixth grader." The Atlantic's Jeffrey Goldberg, in a compelling profile, wrote that while the general wouldn't comment on Trump, "his aides and friends say he found the president to be of limited cognitive ability and of generally dubious character."

Mattis may have had little choice but to dispute these accounts. Bob Woodard doesn't make up quotes, and Jeffrey Goldberg doesn't cite uninformed sources.

In defending his decision not to comment on a president, in an interview with PBS NewsHour's Judy Woodruff (my wife) he invoked the French "duty of reserve," or silence, and cited his predecessor, Secretary Ash Carter, who "has studiously avoided political statements."

Except that he hasn't. In his memoir, released in June, Carter says he couldn't work for Trump, whom he finds "offensive, racist and divisive," noting that in the military, people were fired "for doing even less."

That duty of silence is not the gold standard for retired generals.

There were some crackpots like Curtis LeMay who ended up as the vice-presidential running mate for segregationist George Wallace's presidential campaign. But some of America's greatest, most admired generals — like Matthew Ridgway and James Gavin — spoke out against President Lyndon Johnson's Vietnam War policies.

As for the Marines, one of the most celebrated is the late Gen. David Shoup, who was awarded a Medal of Honor in World War II and later was commandant of the Marine Corps. In retirement, Shoup forcefully opposed Lyndon Johnson on Vietnam, declaring it was "not worth the life or limb of a single American." (I can easily imagine the outrage another great Marine, one of my heroes, now deceased, Col John Glenn, would be expressing about President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE.)

Mattis, as he is quick to point out, is far from perfect. In his final years as an active duty general, he was a big promoter of Theranos, a company that supposedly invented a special blood test that would dramatically improve treatments. In retirement, he served on the Theranos board. It was, as revealed by the Wall Street Journal, a multi-billion dollar scam and is now bankrupt.

That doesn't detract from Mattis’s extraordinary service, the fact he’s an exemplar of integrity and forthrightness. Yet in that NewsHour interview he said he would say so if he thinks a president isn't fit to be commander in chief. So Trump is fit? "No, I'm not saying that. I don't make political assessments."

Mattis says he will speak out about the Trump administration "when the time's right."

That time is soon.