Washington (CNN) Former Secretary of Defense James Mattis defended his reluctance to openly criticize the Trump administration Tuesday but also did not rule out speaking candidly before the 2020 election, telling CNN's Christiane Amanpour that "there will come a time when it is right" for him to do so.

In his first CNN interview since resigning eight months ago , Mattis suggested that he continues to weigh a responsibility to speak out with a duty of silence he believes heowes current officials as they navigate a myriad of geopolitical challenges, raging from Iran to Syria to North Korea.

For the moment, Mattis said that he does not think he adds anything by "representing contrary views" but made it very clear that "there will come a time when it's right for me to talk about strategy and policy" and that he will know that moment when he sees it.

Pressed on whether that time might come before the 2020 election, Mattis was non-committal.

"You're a military man. Duty and honor are very important in your life and in your career. Do you believe it's your duty to speak about what you know from the inside before the next election?" Amanpour asked.

"Duty and honor absolutely are important. And you don't surrender your oath to support and defend the constitution when you leave active duty. But that said, I don't think now, for a person steeped in the military tradition, in the Defense Department, that I should be speaking up on things that are political assessments," Mattis responded.

Mattis echoed that sentiment later Tuesday while speaking before the Council of Foreign Relations.

"I've led a responsible life. I know what responsibility is," he said. "It's not the right place for the defense establishment to be dictating or somehow influencing, I think, the political associations, the political qualifications of people. That's for the American people."

In an interview with The Atlantic released last week, Mattis did note that his code of silence has an expiration date. "There is a period in which I owe my silence," he said. "It's not eternal. It's not going to be forever."

Duty of silence?

Mattis resigned as President Donald Trump's first defense secretary in late December after the President announced plans to withdraw US troops from Syria. Mattis cited irreconcilable policy differences in a letter to Trump that took many in Washington by surprise.

In last week's Atlantic interview Mattis said he owed it to the remaining administration officials to maintain his silence on certain issues.

"If you leave an administration, you owe some silence," he 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."

Mattis explained his thinking behind the degree of discretion he gave Trump.

"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," he said.

But while the retired four-star Marine Corps general has not mentioned Trump by name, he has implicitly criticized his former commander in chief, with whom he sharply disagreed on matters of international engagement and alliances, in a string of recent public statements and interviews.

He also addresses policy matters as they relate to Trump in his forthcoming book, "Call Sign Chaos: Learning to Lead."

Thinly veiled criticism

Since taking office, Trump, a frequent NATO skeptic, has been sharply critical of some members of the alliance, and notably broke with members of the G7 last month in pushing for Russia to rejoin the group.

Since leaving office, Mattis has offered thinly veiled criticism of Trump's perception of world affairs.

In an essay adapted from the book that was published in The Wall Street Journal last week, Mattis rejected Trump's preference for American isolationism, writing that the US is "at increasing risk in the world" when it doesn't embrace its allies.

On Tuesday, Mattis again warned that western democracies are at risk due to the general intolerance of modern politics but would not be drawn into a discussion about the President's own comments and actions.

"Today, we're locked in election cycle, finding reasons not to cooperate, and no democracy can survive that style, it just won't work," he told CNN.

Mattis also appeared to contradict Trump's assessment that ISIS has been defeated though was careful to avoid criticizing the President directly.

Instead, Mattis said he agreed with Secretary of State Mike Pompeo's description of ISIS, saying it "is accurate, they're resurging, and we have to deal with it."

Asked again whether the President was wrong in saying ISIS had been defeated, Mattis conceded that the situation is more complicated than Trump's description.

"It's not the sort of thing you defeat even taking away geography. It's an idea, stronger, more difficult, takes longer," he said.