Mattis' resignation letter was delivered shortly after Trump's decision to announce a unilateral USA withdrawal from Syria and a decision to also leave the 1987 Intermediate-Range Nuclear Forces Treaty.
Plans for the troop withdrawal will be overseen by Deputy Secretary of Defence Patrick Shanahan, who Mr Trump on Sunday said would replace Mr Mattis on Jan 1. While President Obama repeatedly promised to pull all troops out of Afghanistan by 2014, US and North Atlantic Treaty Organisation forces remain there in a predominately training and advising role. "General Mattis did not see this as a problem".
Trump said the United States would remain in Iraq, adding, "In fact, we could use this as the base if we wanted to do something in Syria". "I DO, and it is being fixed!"
On Christmas Eve, the Department of Defense tweeted out a holiday letter from Mattis to the military, which again praised troops for "keeping watch by night" during the season, noting they "carry on the proud legacy of those who stood the watch in decades past".
White House Targets Pelosi Even After Trump's 'Proud' Shutdown Vow
The shutdown is idling hundreds of thousands of federal workers and beginning to pinch citizens who count on some public services. Army soldiers install concertina wire along the United States - Mexico border in Hidalgo, Texas, U.S., November 8, 2018.
Jonathan Ernst/REUTERSPresident Donald Trump and first lady Melania Trump meet USA troops in an unannounced visit to Al Asad Air Base in Iraq, December 26, 2018. During a White House lunch with conservative lawmakers Saturday, Trump discussed his options.
Although the Taliban previously rejected to talk with the Afghan government delegation during the meetings in Abdu Dhabi, but Afghan government says they are keen to send a peace delegation to Jeddah.
Shanahan did not serve in the military.
He earned a bachelor's degree in mechanical engineering from the University of Washington, then a master's in mechanical engineering as well as an MBA from the Sloan School of Management at the Massachusetts Institute of Technology. He spent over three decades working for Boeing, including as vice-president and general manager of Boeing Missile Defence Systems, before moving to the Pentagon as deputy a year ago. Trump called Boeing "a hell of a company" and said Shanahan is a "good buyer" who can eliminate costly overruns on weapons.