Former Secretary of Defense James N. Mattis said he respects a “duty of silence” after leaving President Trump’s administration, but warned Mr. Trump that he won’t stay quiet forever, The Atlantic reported on Thursday.

Mr. Mattis — who submitted his resignation in December after disagreeing with Mr. Trump’s decision to pull troops from Syria — discussed “the French concept of “devoir de réserve” or “the duty of silence” in the Atlantic interview.

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

While the former U.S. Marine Corps general said it’s inappropriate to “endanger the country by attacking the elected commander in chief,” Mr. Mattis said he might not keep that duty of silence forever.

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

Mr. Mattis spoke out this week for the first time since he left the Department of Defense, using excerpts from his new book “Call Sign Chaos: Learning to Lead” to pen an essay in The Wall Street Journal Wednesday, saying he “did as well as I could for as long as I could.”

He also warned Mr. Trump that if he continues his rhetoric against foreign allies, the U.S. will find themselves in “an increasingly lonely position” in the world.

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