James Mattis listens as U.S. President Donald Trump answers questions in Washington in 2018 | Win McNamee/Getty Images Mattis says he owes Trump silence, but won’t keep quiet ‘forever’ ‘If you leave an administration, you owe some silence,’ former secretary of defense says.

Former U.S. Secretary of Defense James Mattis said in a new interview he feels duty-bound to keep quiet his personal opinions of President Donald Trump’s leadership, but revealed his obligation to refrain from criticism of the current commander-in-chief is “not eternal.”

The revered former Marine Corps general, who exited the administration late last year amid a disagreement over Trump's decision to pull U.S. troops from Syria, invoked “the French concept of devoir de réserve” in a conversation with The Atlantic’s Jeffrey Goldberg for a story published Thursday.

“The duty of silence," Mattis explained. "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 continued, arguing that “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.”

Pressed on whether he bears a responsibility to warn Americans about a potentially unfit president, Mattis insisted it was inappropriate to “endanger the country by attacking the elected commander in chief.”

But Mattis also indicated he may soon more vocally challenge Trump or speak out about his time in the president’s Cabinet. “There is a period in which I owe my silence. It’s not eternal. It’s not going to be forever,” he said.

Mattis’ comments to The Atlantic marked the second time this week the former Pentagon chief has hinted at the broader policy disputes with Trump that precipitated his resignation in December 2018.

In an essay published Wednesday in the Wall Street Journal adapted from his forthcoming book, cautioned against the “internal divisiveness” and “tribalism” overtaking American politics.

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