Silence about Trump 'not going to be forever': Former Defense Secretary James Mattis Mattis made the comments in an interview with The Atlantic about his new book.

Ahead of the release of his upcoming book, James Mattis is offering deeply personal insights into his decision to serve and later resign as President Donald Trump's defense secretary and warning about the current state of American politics. And while Mattis is keeping most of his thoughts about the current president private because "I owe my silence," he said in an interview with The Atlantic published Thursday that his silence is "not eternal, it's not going to be forever.”

Mattis, a retired Marine Corps four-star general, served as President Donald Trump's first defense secretary but resigned in December over policy differences, particularly Trump's plans to pull out American troops from Syria.

"I had no choice but to leave” Mattis told The Atlantic in an interview published on Thursday ahead of the release of his new book "Call Sign Chaos: Learning to Lead," available on Sept. 3.

Mattis told The Atlantic that when President Trump rejected his plea to keep U.S. troops in Syria he told Trump “You’re going to have to get the next secretary of defense to lose to ISIS. I’m not going to do it.”

In the interview Mattis was reluctant to answer questions about President Trump’s capacities, inclination’s or character noting that “If you leave an administration, you owe some silence.” Mattis said. “When you leave an administration over clear policy differences, you need to give the people who are still there as much opportunity as possible to defend the country.”

“There is a period in which I owe my silence," said Mattis. But he added that silence is "not eternal, it's not going to be forever.”

“I may not like a commander in chief one fricking bit, 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” said Mattis.

Mattis said he included the text of his resignation letter in his book because “I want people to understand why I couldn’t stay. I’ve been informed by four decades of experience, and I just couldn’t connect the dots anymore.”

"Using every skill I had learned during my decades as a Marine, I did as well as I could for as long as I could," Mattis wrote in an essay for the Wall Street Journal published on Wednesday. "When my concrete solutions and strategic advice, especially keeping faith with our allies, no longer resonated, it was time to resign, despite the limitless joy I felt serving alongside our troops in defense of our Constitution."

In his essay Mattis also pointed out the divisions dominating the political landscape.

"Unlike in the past, where we were unified and drew in allies, currently our own commons seems to be breaking apart," Mattis wrote.

He also warned about what he called "political fratricide" in the nation's capital.

"What concerns me most as a military man is not our external adversaries," he wrote. "It is our internal divisiveness. We are dividing into hostile tribes cheering against each other, fueled by emotion and a mutual disdain that jeopardizes our future, instead of rediscovering our common ground and finding solutions.

"On each of our coins is inscribed America's de facto motto, 'E Pluribus Unum' -- from many, one," he said later. "For our experiment in democracy to survive, we must live that motto."

Mattis said he was shocked when he received a call from Vice President-elect Mike Pence in November 2016 asking if he would meet with Trump in Bedminster, New Jersey, to discuss the defense secretary job.

The former Marine Corps general had retired from the service in 2013, after serving as the head of U.S. Central Command and was doing research at Stanford University's Hoover Institution, where he has since returned.

After his meeting with then president-elect, Mattis wrote, "I figured that my strong support of NATO and my dismissal of the use of torture on prisoners would have the president-elect looking for another candidate."

But to Mattis' surprise, he was nominated to the post -- an assignment that he said he felt "prepared" to do, knowing the job "intimately" from his decades of service in the Marine Corps.

"On a personal level, I had no great desire to return to Washington, D.C," Mattis wrote. "I drew no energy from the turmoil and politics that animate our capital. Yet I didn't feel overwhelmed by the job's immensities. I also felt confident that I could gain bipartisan support for the Department of Defense despite the political fratricide practiced in Washington."

Mattis was confirmed as defense secretary in an overwhelming 98-1 vote by the U.S. Senate.

After serving for nearly two years, he resigned in December, and his deputy Patrick Shanahan stepped into the role in an acting capacity before bowing out following reports of a family history of domestic violence.

Former Army Secretary Mark Esper was formally nominated and confirmed to the job in July.

The White House did not immediately respond to a request for a comment.