Pentagon chief of staff Rear Admiral Kevin Sweeney has resigned, the Department of Defense said Saturday. "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 a statement.

According to Sweeney's bio, he retired as a Rear Admiral from the U.S. Navy in 2014 after 32 years in operational service. In April 2001, Sweeney assumed command of USS Cole in and he was responsible for Cole's restoration and return to fleet operations in April 2002. After that, he reported to the staff of Commander, Theodore Roosevelt Carrier Strike Group as the surface operations officer and then operations officer, and deployed to the Mediterranean Sea in support of Operation Iraqi Freedom.

During Sweeney's time in the private sector, he was Vice President of Track Patch 1 Corporation and served on various boards.

Pentagon chief of staff Kevin Sweeney Department of Defense

When he returned to the Pentagon, Sweeney had served under former Defense Secretary James Mattis, who gave his resignation letter to President Trump on December 20. In his resignation letter, Mattis cited differences with the president. "You have the right to have a secretary of defense whose views are better aligned with yours," Mattis wrote.

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Mattis resigned one day after Mr. Trump abruptly announced the U.S. would be withdrawing troops from Syria.

Mattis initially said he would stay until the end of February, but Mr. Trump said a few days later that Deputy Secretary Patrick Shanahan would take over as acting Defense Secretary on Jan. 1.

One day after Mattis' resignation, Brett McGurk, special presidential envoy for the Global Coalition to Defeat ISIS, submitted his resignation. Sources told CBS News McGurk informed Secretary of State Mike Pompeo that he will accelerate his departure due to a strong disagreement with Mr. Trump about Syria.