WASHINGTON — Jim Mattis, who resigned as Pentagon chief three months ago, is returning to Stanford University.

The Hoover Institution at Stanford announced Tuesday that Mattis will join the public policy think tank to research and write about domestic and international security policy. He was in the same role at Hoover when President Donald Trump nominated him to be secretary of defense in December 2016.

Mattis resigned in December 2018 after a series of policy conflicts with Trump, capped by the president’s decision to pull all U.S. troops out of Syria, a plan he has since partially reversed. Trump has not yet nominated a successor; Patrick Shanahan, who was the deputy secretary during Mattis’ tenure, has been serving as the acting secretary since Jan. 1.

Mattis, who served more than 40 years in the Marines before retiring as a four-star general in 2013, had been working as a national security specialist at Hoover when Trump nominated him to lead the Pentagon.

A Hoover announcement said Mattis will start May 1 as the Davies Family Distinguished Fellow. The announcement quoted Mattis as saying he believes he has an obligation to pass along lessons, for the benefit of future generations.

“The wealth of knowledge and experience of an already extraordinary career has been made even richer by Gen. Mattis’ latest endeavor as secretary of defense, and we are fortunate to once again be beneficiaries of his acumen,” said Tom Gilligan, director of the Hoover Institution.