The Department of Defense's chief of staff Rear Adm. Kevin Sweeney has resigned, according to a statement.

Sweeney said in a brief statement that he planned to return to the private sector, giving no reason for his departure. His exit comes just days after Secretary of Defense James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE, for whom Sweeney previously served, left the agency after announcing his resignation earlier in the month.

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"After two years in the Pentagon, I've decided the time is right to return to the private sector. It has been an honor to serve again alongside the men and women of the Department of Defense," Sweeney said in his announcement.

His resignation also followed that of another top Mattis ally, Pentagon spokeswoman Dana White, who announced her resignation at the end of December.

“I appreciate the opportunity afforded to me by this administration to serve alongside Secretary Mattis, our Service members and all the civilians who support them. It has been my honor and privilege,” White wrote on Twitter last month. “Stay safe and God bless.”

Mattis's resignation was thought to be tied to the surprise announcement from President Trump Donald John TrumpBarr criticizes DOJ in speech declaring all agency power 'is invested in the attorney general' Military leaders asked about using heat ray on protesters outside White House: report Powell warns failure to reach COVID-19 deal could 'scar and damage' economy MORE regarding the withdrawal of U.S. troops from Syria, where coalition forces continue to battle elements of the Islamic State in Iraq and Syria (ISIS) on the ground.

Trump's withdrawal of U.S. forces from Syria was also credited at the time with pushing the resignation of Brett McGurk, the president's top envoy to anti-ISIS coalition forces in the region.

Mattis's former deputy, Patrick Shanahan Patrick Michael ShanahanHouse Armed Services chairman expresses confidence in Esper amid aircraft carrier coronavirus crisis Boeing pleads for bailout under weight of coronavirus, 737 fallout Esper's chief of staff to depart at end of January MORE, began serving as acting Defense secretary at the beginning of January.