Former Secretary of Defense Jim Mattis defended his decision to remain silent about any criticism of President Donald Trump, citing an American tradition and a duty to keep his own counsel on matters of politics.

In an interview with Atlantic’s Jeffery Goldberg, Mattis repeatedly rejected his attempts to get him to criticize the president.

“I didn’t cook up a convenient tradition here,” he said. “You don’t endanger the country by attacking the elected commander in chief. 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.”

Citing the French concept of “devoir de réserve,” Mattis referred to the “duty of silence.”

“If you leave an administration, you owe some silence,” he explained.

Mattis was critical of the “poisonous” attempt by some to destroy the duly elected president.

“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,” he said.

Mattis said he was duty-bound to silence at the moment about Trump but admitted it would not last forever.

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

The article also reveals Mattis was convinced Trump would lose to Islamic State by withdrawing American troops from Syria and abandoning their allies in the country.

“You’re going to have to get the next secretary of defense to lose to ISIS,” he said. “I’m not going to do it.”

American troops remain in Syria, despite Trump’s expressed desire to bring them home.

Mattis is releasing a new book Call Sign Chaos, an autobiography of his military career that ended with his resignation from the Department of Defense under President Donald Trump. It is also a treatise on leadership and a defense of his views on foreign policy. It is not expected to be a book that is critical of Trump.

In his resignation letter, Mattis alluded to his differences with Trump, but it remains the strongest dispute with the president.

“Because you have the right to have a Secretary of Defense whose views are better aligned with yours on these and other subjects, I believe it is right for me to step down from my position,” he wrote.