Pentagon chief of staff resigns

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 MattisJames Norman MattisOvernight Defense: Pentagon chief under investigation over Boeing ties | Trump uses visual aids to tout progress against ISIS | Pentagon, Amnesty International spar over civilian drone deaths Pentagon watchdog probing whether acting chief boosted Boeing Overnight Defense: Judge says Trump can't implement transgender policy | Trump floats admitting Brazil to NATO | Mattis returning to Stanford 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 TrumpDonald John TrumpSenate GOP budget ignores Trump, cuts defense Trump says he'll nominate Stephen Moore to Fed White House: ISIS territory in Syria has been 100 percent eliminated 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 ShanahanPatrick Michael ShanahanWhite House: ISIS territory in Syria has been 100 percent eliminated Overnight Defense: Top Marine warns border deployment could hurt readiness | McSally aims for sexual assault reforms in defense bill | House to vote on measure opposing transgender ban | New warning over F-35 sale to Turkey On The Money: Trump rolls dice on uncertain economy | 737 crisis tests Boeing's clout in Washington | Watchdog group pushes 2020 candidates for 10 years of tax returns MORE, began serving as acting Defense secretary at the beginning of January.