In his resignation letter to Trump, however, Rood indicated it was the commander in chief who wanted him out.

“It’s my understanding from [Defense Secretary Mark] Esper that you requested my resignation,” Rood wrote to Trump. “Senior administration officials appointed by the President serve at the pleasure of the President, and therefore, as you have requested, I am providing my resignation effective February 28, 2020."

James Anderson, who's filling in for the No. 2 position in the Pentagon's policy shop, will take over Rood's job until a permanent replacement is named, DoD spokesperson Alyssa Farah said in a statement.

Rood's exit from the administration was first reported by CNN. A former DoD official said Rood was asked to resign due to "unhappiness with him for some time." Foreign Policy reported in December that several officials who have left the Pentagon cited "a toxic work environment" under Rood.

Anderson, Rood's temporary replacement, had been "performing the duties of" Rood's deputy since David Trachtenberg retired last summer. Anderson was confirmed last year as assistant secretary for strategy, plans and capabilities.

Last May, Rood certified to Congress that Ukraine made enough progress in combating corruption that it should receive $250 million in military assistance. The money was withheld by the White House over the summer, a move that led to Trump's impeachment by the House. The administration claimed the aid was withheld over concerns with corruption in Kyiv, though House Democrats tied the move to Trump's efforts to pressure the nation to dig up dirt on his political opponents.

The Pentagon’s chief spokesperson, Jonathan Hoffman, didn’t elaborate on why Trump wanted Rood gone or whether Esper recommended removing him, telling reporters on Wednesday that the policy chief’s resignation letter “speaks for itself.” Hoffman also said it was "speculative" to link Rood’s departure to his certification of the Ukraine aid.

“The president has the opportunity and the ability to have the team that he wants to have in policy positions throughout the federal government,” Hoffman said at a Pentagon press briefing. “John had the opportunity to serve in a senior position at the department … as an appointee of the president, but the president can make a decision to go a different direction. So I’m not going to speculate on the motivations.”

Rood also led several other initiatives at the department. He spearheaded the Pentagon's long-delayed final blueprint for future U.S. missile defense capabilities. The review had been underway before Rood took over, but he reportedly elected to redo the report upon his confirmation.

Rood was also responsible for the Pentagon's about-face on a proposed crackdown on sales by U.S. companies to sanctioned Chinese telecommunications firm Huawei. Rood overruled concerns from Pentagon research and engineering officials that further restricting exports to Huawei would hurt U.S. companies' competitive edge.

Prior to joining the Pentagon in 2018, Rood held senior nonproliferation posts at the State Department and National Security Council. He was also a deputy assistant secretary of Defense for forces policy as a well as a policy adviser to then-Republican Sen. Jon Kyl.

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Most recently, Rood was a senior vice president at Lockheed Martin's international arm. Rood was easily confirmed by the Senate despite a bumpy confirmation hearing where late-Armed Services Chairman John McCain and Sen. Elizabeth Warren (D-Mass.) grilled him over whether he'd seek waivers to his two year ethics agreement by which he recused himself on matters that could affect Lockheed's bottom line.

Rood is just the latest in a raft of senior departures, including the Pentagon's acting personnel chief, top Asia policy official, No. 2 intelligence official, and Defense Advanced Research Projects Agency director.

Trump, meanwhile, still has not nominated a replacement for fired Navy Secretary Richard Spencer, despite announcing ambassador to Norway Kenneth Braithwaite as his pick late last year.

Bryan Bender contributed to this report.

