Former Secretary of Defense James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE is releasing a book this summer about his 43 years in the military, The Associated Press reported Tuesday.

Mattis, however, warned that the book would not be a scathing exposé about his time in the administration.

“Call Sign Chaos: Learning to Lead” will be an “expansive” account of Mattis’s military career, named as a tribute to his call sign “Chaos” while he lead the 7th Marine Regiment in the mid-1990s.

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A spokesperson for Random House publishing told AP that the book was under contract before Mattis became President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE’s Defense chief in early 2017.

“My purpose in writing this book is to convey some of the lessons I learned in 43 years of service for those who might benefit, whether in the military or in civilian life,” Mattis, 68, said in a statement. “I’m old-fashioned: I don’t write about sitting Presidents, so those looking for a tell-all will be disappointed. I want to pass on the lessons and experiences that prepared me for challenges I could not anticipate, not take up the hot political rhetoric of our day.”

Mattis, a retired four-star general, announced his departure from the Pentagon in December following Trump’s surprise announcement that the administration would withdraw U.S. troops from Syria, where coalition forces continue to fight elements of ISIS on the ground.

He wrote a scathing resignation letter to Trump, saying that the president deserved a general “whose views are better aligned with yours.”

The president first issued a cordial statement on the departure, writing on Twitter that Mattis “will be retiring, with distinction, at the end of February,” and praising the former general for his “tremendous progress” at the Defense Department.

Trump later changed his tune following news around the critical resignation letter, saying that Mattis’s performance at the Pentagon was “not too good.”

“I didn’t like the job he was doing. I wasn’t happy with it,” Trump said of the former chief. “I wasn’t happy with the — I got him more money than the military has ever seen before. And I wasn’t happy with the job that he was doing at all. And I said it’s time.”

A number of former White House officials who served in the Trump administration or campaign have released books about their experiences, but Mattis’s book is set to be the first title from a former Cabinet member.

The president’s eldest son, Donald Trump Jr. Don John Trump'Tiger King' star Joe Exotic requests pardon from Trump: 'Be my hero please' Zaid Jilani discusses Trump's move to cancel racial sensitivity training at federal agencies Trump International Hotel in Vancouver closes permanently MORE, will also be releasing a book later this year.

“Call Sign Chaos: Learning to Lead,” co-written by Bing West, will be published July 16 by Random House.