The Pentagon's top policy official John Rood resigned on Wednesday at the request of President Donald Trump, according to a copy of his resignation letter obtained by CNN.
John Rood, Under Secretary of Defense for Policy at the Pentagon, had lost support among senior national security leadership, one source said, and officials told CNN that he broke with the administration on several key issues.
"It is my understanding from Secretary Esper that you requested my resignation from serving as Under Secretary of Defense for Policy. 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," Rood wrote in his letter to President Donald Trump, dated Wednesday.
Trump also confirmed Rood's departure in a tweet on Wednesday.
"I would like to thank John Rood for his service to our Country, and wish him well in his future endeavors!" Trump wrote, also sharing a story from Bloomberg News which indicated that Rood "faced pressure to resign from some who lost confidence in his ability to carry out Trump agenda."
Defense Department press secretary Alyssa Farah said in a statement that "Dr. James Anderson, the current senior official Performing the Duties of the Deputy Under Secretary of Defense for Policy will take over the responsibilities of Undersecretary Rood until a permanent replacement is appointed by the President and confirmed."
Officials tell CNN that Rood has differed with the administration on a number of issues including Afghanistan and Ukraine. Officials have said Rood often was perceived as not embracing some of the changes in policy the White House and senior Pentagon officials wanted.
One official said some examples of Rood's differing views from some of Trump's key policy stances included being skeptical about peace talks with the Taliban as well as the administration decision to scale down military exercises with South Korea during talks with North Korea and him pushing for a more aggressive approach to Russia by supporting Ukraine.
Rood is the Pentagon's top policy official and oversees aspects of the Pentagon's relationship with US allies and partners.
He was involved in certifying to Congress that Ukraine had embarked on significant reforms to justify its receipt of $250 million in security assistance.
That certification undermined one of the justifications -- concerns about corruption in Kiev -- that some members of the Trump administration made to defend blocking aid to Ukraine.
Hours after Trump's July 25 phone call with Ukrainian President Volodymyr Zelensky, a conversation that was at the center of impeachment proceedings, Rood emailed Secretary of Defense Mark Esper -- who had been in the job two days -- informing him about an upcoming deputies meeting, "to discuss the President's concern about endemic corruption in Ukraine and his reported view that US should cease providing security assistance," according to emails reviewed by CNN.
Rood notes in his email to the secretary that "placing a hold on security assistance at this time would jeopardize this unique window of opportunity and undermine our defense priorities with a key partner in the strategic competition with Russia."
As head of policy at the Pentagon, Rood helped oversee implementing the Trump administration's National Defense Strategy, which placed a greater emphasis on countering China and Russia, as well as overseeing the Nuclear Posture Review, which called for changes to the US nuclear arsenal, including the addition of new low-yield nuclear weapons.
"I would like to thank John Rood for his service to the Department," Esper said in a statement Wednesday. "John has played a critical role on a wide range of DoD issues including modernizing our nuclear deterrence capability, efforts to increase burden sharing by our NATO allies, our Missile Defense Review and implementing the National Defense Strategy. I wish him all the best in his future endeavors."
Rood joined the administration in January 2018 serving under then-Defense Secretary James Mattis. He has had numerous policy jobs at the Pentagon in previous administrations and also previously worked for the CIA as an analyst. He has also held senior roles with major defense contractors Lockheed Martin and Raytheon.
This story has been updated with additional information.