US Army Secretary Dan Driscoll has submitted his resignation after about 18 months in the role.
His departure was confirmed by CBS, the BBC’s US media partner, which reported it came after months of conflict between him and Defence Secretary Pete Hegseth.
The youngest-ever army secretary has often been referred to as President Donald Trump’s “drone guy”.
Driscoll, 41, has been “highly effective” in advancing Trump’s agenda “to Make America Strong Again” and provided “outstanding leadership” during military operations, the White House said in a statement given to CBS.
The BBC has contacted the White House, Pentagon and Army for comment.
Driscoll is the latest in a string of recent high-profile military leaders to depart, including Navy Secretary John Phelan who abruptly quit in April.
Other notable departures include Army Chief of Staff Randy George, Army Gen David Hodne and Maj Gen William Green.
Since entering the Pentagon, Hegseth has fired more than a dozen senior military officers, including the chief of naval operations and the Air Force’s vice-chief of staff.
The details of the behind-the-scenes friction between Driscoll and Hegseth are unclear but CBS notes that the army secretary is good friends with George, who was reportedly asked by Hegseth to resign.
The post of army secretary is the top civilian leader of the US Army, directing active duty troops, along with the National Guard and reserve soldiers.
Driscoll previously served in the Army and was an officer in 2007 before leading a cavalry platoon and deploying to Iraq for several months in 2009.
As army secretary, Driscoll became known as Trump’s “drone guy” due to his embrace of emerging battlefield technologies.
He took on a leading role last year in talks aimed at ending the Russia-Ukraine war, taking on foreign trips and meeting with Ukrainian President Volodymyr Zelensky.
He’s also a close ally and friend of Vice-President JD Vance, whom he met at Yale Law School.
During his tenure as head of the US Army, he was a key figure in Trump’s deployment of the National Guard to US cities.
He also took over the role of acting director of the Bureau of Alcohol, Tobacco, Firearms and Explosives.
