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– John Rood, the Defense Department's top policy official, is the latest member of President Donald Trump's national security team involved in the Ukraine matter to be ousted from the government.

Rood, the undersecretary of defense for policy, will step down at month's end, the department's press secretary, Alyssa Farah, said Wednesday.

A Defense official said Trump had told Defense Secretary Mark Esper that he wanted Rood out. And Rood made it crystal clear that he had been pushed out. "It is my understanding from Secretary Esper that you requested my resignation," Rood said in the letter to the president dated Wednesday. "Therefore, as you have requested, I am providing my resignation."

Trump said in a tweet: "I would like to thank John Rood for his service to our Country, and wish him well in his future endeavors!"

Rood was part of the team at the Defense Department that told Congress last year that Ukraine had made the necessary reforms to justify sending the country $250 million in promised security assistance. The certification was widely viewed as undermining a key argument Trump's defense team made during his impeachment battle: that Trump withheld the aid because he was concerned about corruption in Ukraine.

Trump was impeached by the Democratic-controlled House but acquitted by the Republican-controlled Senate. Since his acquittal, the president has moved swiftly to purge administration officials whose presentation of events did not align with his own.

In the days after his acquittal, Trump fired two of the most prominent witnesses in the House inquiry against him: Gordon Sondland, the ambassador to the European Union, and Lt. Col. Alexander Vindman, a decorated Iraq War veteran on the National Security Council staff.

Another Pentagon spokesman, Jonathan Hoffman, pushed back against speculation that Rood was pushed out because of his role supporting the security aid to Ukraine. "I have no information that would lead me to that conclusion," Hoffman said.

Rood's departure was not entirely unexpected; he and Esper were known to clash early in their careers, and Esper was expected to fire him when he became Defense Secretary last year. Esper said Rood had "played a critical role on a wide range of DOD issues."

James Anderson, the acting deputy undersecretary of defense for policy, will be taking over until a replacement is appointed, the department said.

Rood's departure accelerates the hollowing out of civilian leadership at the Pentagon, noted Derek Chollet, a former assistant secretary of defense.

Other departures include Eric Chewning, Esper's chief of staff; Randy Schriver, the assistant secretary of defense for Indo-Pacific security, and Navy Secretary Richard Spencer. The departure of so many civilians means that the Pentagon is increasingly in the hands of two entities: Esper, who has adhered closely to the policies of Trump; and the senior military leadership under Gen. Mark Milley, chairman of the Joint Chiefs of Staff.