Kevin Sweeney has resigned as Pentagon chief of staff after serving the defense secretary for two years.
"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 a short statement posted on the Department of Defense website late Saturday night.
A knowledgeable source told CNN's Jake Tapper that the White House forced Sweeney out. The source did not provide further information about the reason.
Sweeney, who retired as a rear admiral in 2014, served under former Defense Secretary James Mattis, who announced his resignation December 20 on the heels of President Donald Trump's plans to withdraw troops from Syria.
"Because you have the right to have a Secretary of Defense whose views are better aligned with yours on these and other subjects, I believe it is right for me to step down from my position," Mattis wrote in his letter to the President.
Trump said he moved Mattis' departure date up to January 1, two months earlier than originally planned.
Mattis is the latest senior administration to leave Trump's Cabinet. Jeff Sessions was pushed out as attorney general the day after the midterm elections in November, Rex Tillerson was unceremoniously fired as secretary of state in March and national security adviser H.R. McMaster was replaced.
Chief Pentagon spokeswoman Dana White announced last month that she's leaving her post, a move that while it coincided with Mattis stepping down, came amid an internal Defense Department investigation into her conduct.