Former four-star general, Defense Secretary James Mattis has written a new book, “Call Sign Chaos: Learning to Lead,” about his 40 years in the Marines, and his philosophy on leadership. When asked questions regarding his relationship with the Commander in Chief, President Trump—Mattis, who began writing his book in 2013, shortly after his retirement from the Marines, was not prepared to trash the president—despite promptings from interviewers. The focus of his book is passing on lessons he has learned, hoping his experiences will help others.

Philippine ambassador to the U.S., Jose Manuel Romualdez (L) with U.S. Defense Secretary James Mattis at two of the Bells of Balangiga at F.E. Warren Air Force Base in Cheyenne, Wyo. Three church bells seized by American troops as war trophies more than a century ago arrived in Manila on Tuesday, Dec. 11, 2018. (Mead Gruver/AP Photo, File)

Mattis served under the Trump administration as the president’s first defense secretary and is speaking out with the release of his new book.

Fox & Friends” co-host Steve Doocy asked Mattis about his two-year tenure serving under President Trump, before resigning from his position. “Do you talk about working as defense secretary for Donald Trump in the book,” he asked, adding, “aAlot of people would like to hear about it. From where we sat it looked like a tense relationship.”

Mattis was quick to correct Doocy,“No, it wasn’t a tense relationship,” adding, “The president is a forthright man and so am I. I would meet with him weekly. There was nothing going on that I wasn’t open with him about. That’s the way I deal with my boss anywhere I have ever been.”

“Because he is the boss, right?” Doocy said. “He was elected by the American people,” Mattis replied.

Mattis was in charge of Central Command and oversaw the U.S. wars in Iraq and Afghanistan under President Obama, before serving under President Trump.

Fox co-host Doocy questioned Mattis about his return to duty from retirement under President Trump, and he replied, “I did. I was enjoying life,” he said. “But when you are asked to serve, Republican or Democrat, don’t start wringing your hands and asking what to do. Just roll up your sleeves if you are prepared, go to work and do your best. It’s very, very simple.”

President-elect Donald Trump and retired Marine Corps Gen. James Mattis in bedminster, N.J., on Nov. 19, 2016. (Katz/Shutterstock)

In an interview with Judy Woodruff for PBS, Mattis was asked, “Are you confident this is a president who can be trusted with the nuclear codes, a fateful responsibility?” Mattis replied, “Yes.” “You know, the responsibility that lies—and that’s a very grave one, I have not heard anything that would indicate there was some responsibility there. … I realize that we have an unusual president and he talks openly about many things. But at the same time, in the privacy of the office, he has to deal with the reality of competing factors. And I would bring the grim realities of war into that office.”

Mattis also was asked if he would speak out if he thought the president wasn’t fit for the role as commander in chief, he replied, “Yes” he would speak out.