(FILES) In this file photo taken on August 09, 2018 US Vice President Mike Pence (2nd-L) arrives to speak about the creation of a new branch of the military, Space Force, alongside US Secretary of Defense Jim Mattis (L) and Deputy Secretary of Defense Patrick Shanahan (R) at the Pentagon in Washington, DC. - Donald Trump on December 23, 2018 announced he would replace Defense Secretary Jim Mattis with his deputy Patrick Shanahan, days after the outgoing Pentagon chief quit while citing key policy differences with the US president. "I am pleased to announce that our very talented Deputy Secretary of Defense, Patrick Shanahan, will assume the title of Acting Secretary of Defense starting January 1, 2019," the Republican leader tweeted, accelerating Mattis's planned departure by two months. (Photo by SAUL LOEB / AFP)