Jim Anderson, the acting undersecretary of defense for policy at the Pentagon, resigned from his position Tuesday, just one day after President Trump fired Secretary of Defense Mark Esper, a senior U.S. defense official told Fox News.

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“I am particularly grateful to have been entrusted with leading the dedicated men and women of Policy, who play a key role in our Nation’s security,” Anderson wrote in his resignation letter, which was first obtained by Politico. “Now, as ever, our long-term success depends on adhering to the U.S. Constitution all public servants swear to support and defend.”

Anderson held the position for barely nine months after John Rood was ousted from the position in February for perceived disloyalty to Trump. 

Sources told Fox News that Anderson's leave paves the way retired Brig Gen. Anthony Tata, a former Fox News guest, to serve in the acting role.

Tata was nominated to the position earlier this year, but the Senate ceased confirmation hearings even before they began due to a series of Islamophobic tweets that have since been removed from social media.

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Tata will be “performing the duties of” the undersecretary job, rather than holding the “acting” title. Officials who carry the “acting” title have more authority than those who are “performing the duties of” the job, according to reports by the Associated Press. 

Sources also told Fox News that more firings and resignations are expected at the Pentagon in the coming weeks, as the White House tries to insert loyalists into key positions.

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On Monday, Trump appointed Christopher Miller, the director of the National Counterterrorism Center, to take over Esper's position. A day later, the president named White House National Security Council official and former congressional aide, Kash Patel, as Miller's chief of staff, according to reports by the Wall Street Journal