Former Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE said he had “no choice but to leave” the Trump administration in an interview with The Atlantic, but was reluctant to share additional details about his acrimonious departure.

Mattis pointed to his resignation letter, which made clear he had differences with 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, in explaining his decision to leave after nearly two years as Defense secretary.

“That’s why the letter is in the book. 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,” Mattis said.

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His first book, “Call Sign Chaos: Learning to Lead,” will be published on Sept. 3. Early reviews have suggested it will not be a tell-all of his time working for Trump.

The general said it would not be appropriate for him to speak more loudly about any differences he might have had with Trump.

“If you leave an administration, you owe some silence," he said, telling The Atlantic that his decision follows the French concept of “devoir de réserve,” or “duty of silence.”

"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,” said Mattis.

“They still have the responsibility of protecting this great big experiment of ours. I know the malevolence some people feel for this country, and we have to give the people who are protecting us some time to carry out their duties without me adding my criticism to the cacophony that is right now so poisonous.”

Mattis stepped down from the administration in December, issuing a letter that explained some of his differences. While he initially was to stay on as Defense secretary until the end of February, Trump sped the date up to Jan. 1.

Mattis's resignation came after Trump decided to withdraw troops from Syria and as he pushed for a withdrawal from Afghanistan. Mattis had urged Trump to keep troops in Syria and resigned after his pleas were rejected.

Mattis told The Atlantic he may at some point speak more openly about his feelings.

“There is a period in which I owe my silence,” he told the outlet. “It’s not eternal. It’s not going to be forever.”