Washington (CNN) With Defense Secretary James Mattis on his way out two months earlier than planned, Deputy Defense Secretary Patrick Shanahan will take the reins of the government's largest and most complex agency with no military and foreign policy experience.

Shanahan will lead the Pentagon on an interim basis until President Donald Trump names a nominee to be his new defense secretary. Shanahan will be charged with carrying out the President's orders of withdrawing US troops from Syria, and possibly Afghanistan. A defense official confirmed to CNN Monday that Shanahan has canceled his holiday travel and is heading back to Washington.

Patrick M. Shanahan, deputy secretary of defense, poses for his official portrait in the Army portrait studio at the Pentagon in Arlington, Virginia, July 19, 2017.

A former corporate executive, Shanahan's portfolio at the Department of Defense has been narrow with no experience in international affairs or with counterterrorism.

Past Pentagon chiefs have served in the role without prior military service, with Ashton Carter -- President Barack Obama's last secretary of defense -- as the most recent example.

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