In recent days, as Jim Mattis prepares to release a book on his time as a Marine general and US defense chief, he has continued to speak in veiled terms about his time serving President Donald Trump.

Despite his reticence, however, Mattis' true feelings about the commander-in-chief do seem to be bleeding through.

Visit Business Insider's homepage for more stories.

WASHINGTON (AP) — Try as he might, Jim Mattis can't seem to hide his real feelings about Donald Trump — that the president is leading the world's most powerful nation down a dangerously wrong path.

Mattis, the retired Marine general who resigned as defense secretary last December in a military policy dispute with Trump, says he owes the nation public silence while his former boss remains in office.

Yet the comments Mattis is making as he promotes his new book suggest a strong, if implicit, message: Trump's leadership is diminishing America.

From the day he accepted Trump's offer to lead the Pentagon, Mattis knew his views didn't align entirely with those of the president-elect, particularly on what Mattis considers a central pillar of American global power and influence: respect for allies. Trump often denigrates allies, calling them ingrates and freeloaders.

Mattis, who spent more than four decades in the Marines, is a former NATO supreme allied commander. Strengthening alliances was No. 2 on his list of strategic priorities as defense secretary, behind only his push to restore what he saw as America's eroding military edge.

Nations with allies prosper, Mattis likes to say, while those without them wither. Trump prefers to largely go it alone, America first.

Trump at a rally with James Mattis, his pick for defense secretary. AP

During his two-year tenure at the Pentagon, Mattis was consistently circumspect. He shied from news cameras, concerned that any utterance could offend his boss or amplify the daylight between the two men on any number of issues. To preserve his influence, he felt he must hold his tongue.

Nine months after resigning, Mattis still won't spell out his views on Trump, even when pushed in recent interviews lined up to discuss his book, "Call Sign Chaos: Learning to Lead," to be released Tuesday.

He is leaving it to others to interpret his dancing around the question he knows many Americans would like him to answer: Does he think Trump is fit to lead for another four years?

"You don't endanger the country by attacking the elected commander in chief," he told Jeffrey Goldberg for a portrait in The Atlantic that sums up Mattis as "the man who couldn't take it anymore."

Mattis, known as a man who is nothing if not calculating, then came as close as he ever has to putting a verbal knife in Trump.

"I may not like a commander in chief one fricking bit," he said, "but our system puts the commander in chief there, and to further weaken him when we're up against real threats — I mean, we could be at war on the Korean peninsula every time they start launching something."

A companion concern that Mattis has expressed, sometimes obliquely, is that the political climate in which Trump was elected in 2016 has grown even more divisive, to the point of endangering the future of what Mattis likes to call America's experiment in democracy.

"We all know we're better than our current politics," he wrote in an essay for The Wall Street Journal on Wednesday. "Tribalism must not be allowed to destroy our experiment."

Thus, Mattis is sticking with his favorite understated description of a president who defies convention, sows confusion and sometimes abandons truth.

Mattis with Trump during a meeting with military leaders at the White House, October 23, 2018. Win McNamee/Getty Images

"He's an unusual president, our president is," Mattis told correspondent David Martin in an interview for "CBS Sunday Morning." ''And I think that especially with the — just the rabid nature of politics today we've got to be careful. We could tear this country apart."

Critics say Mattis is using veiled attacks on Trump to preserve his own legacy after having chosen to serve in an administration that injected politics into the military in ways Mattis himself was known to abhor. Others say he is doing his best to walk a fine line between explaining himself and violating an unwritten rule: Military officers don't attack a president in office.

Retired Marine Gen. James Jones, whose long career overlapped Mattis's and who later served as national security adviser to President Barack Obama, credits Mattis with a "high-minded" approach.

"He always took the high road," Jones said. "Mattis doesn't take cheap shots."

In his essay for The Wall Street Journal, which was adapted from his book, Mattis described his view of what makes a good leader, which by implication suggested it is not the Trump model.

"Wise leadership requires collaboration; otherwise it will lead to failure," he wrote, adding, "A polemicist's role is not sufficient for a leader. A leader must display strategic acumen that incorporates respect for those nations that have stood with us when trouble loomed."

It was that Mattis measure of leadership — loyalty to allies — that triggered his decision to quit last December. By then it was only a question of time, since Trump has begun publicly mocking Mattis, and Mattis had become upset with a growing list of Trump decisions, including naming Army Gen. Mark Milley to be chairman of the Joint Chiefs of Staff. Mattis had recommended another general.

One day after Trump announced he was pulling all US troops out of Syria, where they were partnering with local Syrians to fight the Islamic State, Mattis tried but failed to change Trump's mind. So, he resigned.

"You're going to have to get the next secretary of defense to lose to ISIS. I'm not going to do it," Goldberg quoted Mattis as telling Trump. Trump soon turned on Mattis, calling him a failure. He said falsely that he had fired Mattis.

"What's he done for me?" Trump said Jan. 2. "How had he done in Afghanistan? Not too good. I'm not happy with what he's done in Afghanistan, and I shouldn't be happy."