Defense Secretary Lloyd Austin tapped a senior State Department adviser to be his new chief of staff.

Derek Chollet has held senior policy roles at the White House, the Pentagon, and the State Department, and he will be heading back to the Department of Defense starting next month, Austin announced Monday morning.

“He is one of the most distinguished, far-sighted, and skillful national-security practitioners of his generation, and I am grateful to him for taking on this key assignment at such an important moment,” the secretary said in a statement. Secretary of State Antony Blinken said he has “depended on Derek Chollet’s wise counsel and steady hand in helping navigate the toughest issues the United States faces.”

Chollet served as the U.S. assistant secretary of defense for international security affairs from 2012 to 2015, and prior to that, he worked at the White House as special assistant to the president and senior director for strategic planning on the National Security Council staff.

Chollet was nominated last year to become the undersecretary of defense for policy, a position that requires Senate confirmation. His nomination was opposed by some Republicans, and he has not been confirmed.

He will be taking over for Kelly Magsamen, who will be leaving the Pentagon at the end of June. She became more well known earlier this year during Austin’s secretive hospital stay in January.

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Chollet will be replaced at the State Department by Tom Sullivan, who is the deputy chief of staff for policy.

“Tom has an unmatched depth of knowledge about how national security policy is formulated and implemented, one that he brings to bear every day on behalf of the American people,” Blinken said.