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 joining the D.C.-based business advisory firm The Cohen Group, according to a copy of the announcement reported by Politico on Sunday.

The group said Mattis will join as senior counsel in October. The announcement is dated to be released Monday.

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“Jim Mattis is a national treasure, a leader known throughout the world for his strategic judgment as well as his unyielding principles,” former Defense Secretary Bill Cohen, Chairman of The Cohen Group, said in the announcement.

"We are honored to be working with him, and our clients will benefit from his knowledge and guidance," Cohen added.

Mattis served in the Trump administration from January 2017 to December 2018.

In an interview with The Atlantic last month, Mattis said he had "no choice but to leave" the administration.

Though he was reluctant to discuss details of his time at the Trump White House, he pointed to his resignation letter to illustrate the reason he left.

His resignation letter followed Trump's decision to withdraw troops from Syria.

Mattis's first book, "Call Sign Chaos: Learning to Lead," was published earlier this month.