While famously media shy during his time in government, former Secretary of Defense James Mattis offered a brief glimpse into his thoughts on today’s political environment in an essay and interview last week, but the appearance left several questions unanswered.

These rare media appearances are not random, as Mattis will be appearing at several events in the coming days to promote his new book, Call Sign Chaos: Learning to Lead. The two pieces delve into why the former Marine Corps general chose to serve as secretary of defense and why he resigned. There are some new details about the journey , but his thoughts on several key points remain unclear. One common theme is his desire to stay above the fray when it comes to politics.

“He felt as a Marine, when he was asked to serve the country there was no question he was going to serve. Period. He felt he was capable of doing it and it never occurred to him to say no,” Bing West, Mattis’ co-author, told the Washington Examiner. “He was asked by the commander in chief to do a mission, he said I’m going to do the mission.”

But Mattis resigned less than two years later, citing policy differences with President Trump. “I had no choice but to leave,” Mattis told the Atlantic.

According to his resignation letter, Mattis left due to concerns regarding the administration’s relationship with key military allies. While he does not accuse Trump of disregarding them outright, he did say the president has “the right to have a Secretary of Defense whose views are better aligned with yours.”

Behind the scenes, it was Trump’s decision to pull U.S. troops out of Syria that was the final straw for the former Marine Corps general.

“You’re going to have to get the next secretary of defense to lose to ISIS. I’m not going to do it,” Mattis reportedly told Trump.

Trump did not pull all U.S. troops out of Syria, and while experts are still concerned about ISIS as a threat, there is no indication that the United States or its partners are necessarily losing. Trump is also known to change his mind on policy issues on occasion, something Mattis might have expected. Most recently, Trump called off a retaliatory strike against Iran in June after Iranian forces shot down a U.S. drone over the Persian Gulf due to his belief that the casualty count would not have been proportional.

“It's all about timing. We were much more effective at killing ISIS with Mattis and Trump at the helm than in the previous years. But to keep the caliphate from forming again, we need to ensure we have a coalition in place to keep at the work,” Rebeccah Heinrichs, a senior fellow at the Hudson Institute, told the Washington Examiner. “And believe me, I want us to have as small a footprint as possible in the Middle East. It's sand and death, as the president called Syria. We need to make a serious shift in resources to the Indo Pacific.”

Throughout his interview and essay, Mattis focuses on the importance of the military staying above the fray of politics. His replacement, Secretary of Defense Mark Esper, reiterated this point in his first ever press briefing as secretary on Wednesday.

“You know, as I answered to Congress during my nomination hearing … my commitment is to keep this department apolitical,” Esper told reporters. “And I believe the best way to do that beings with the chairman [of the Joint Chiefs of Staff] and I behaving in an apolitical way.”

While that may be easier for the military rank-and-file, its significantly more difficult for an appointed secretary.

“The secretary of defense is a political leader. He’s not apolitical,” Lt. Gen. Thomas Spoehr told the Washington Examiner in an interview. “They’re in a different category than the chairman of the Joint Chiefs of Staff or the chief of staff of the Army, that’s not commonly understood.”

Mattis credits his 40 years in the Marine Corps for much of his knowledge, but Spoehr noted that there is one thing the military doesn’t necessarily prepare someone to navigate Washington.

“The military career doesn’t prepare you well for politics, lets just put it that way,” Spoehr said. “Things are much more defined, there’s a process, there’s a hierarchy, typically people do what they are told to do. And in politics, all bets are off. People have a different set of motivations, which is hard for a career military officer.”

Mattis’ refusal to weigh in on Trump directly is one of the aspects of his military background he has maintained even after leaving office. This devoir de reserve, as Mattis put it, “the duty of silence” when you leave an administration, has frustrated his critics who say he should speak out.

“If Mattis thinks the president of the United States is a no-kidding threat to the country, he should say so. I think implying that he thinks that without hearing it from him is dishonest,” Heinrichs said.

Several former Trump administration staffers haven’t followed Mattis’ example, but Spoehr said there is a certain nobility in his staying silent.

“I think him saying that, maybe he’s hoping to restore that trust that the president can have in his advisers that [they] aren’t going to say something that’s immediately or later going to get repeated in the first book they write when they get out of the administration.”

Mattis also noted, though, that his period of silence is “not eternal.”