Trump has issued a further series of statements since then criticizing Mattis, saying Mattis did not see a problem with the United States subsidizing the militaries of rich countries, or allowing them to "take total advantage of the United States and our TAXPAYERS, on Trade".
Jim Mattis has "no anger" after Trump forced him out early, Mattis' brother has revealed.
Shanahan did not serve in the military.
Names that are quietly beginning to surface include some of the civilian leaders of the military services such as Air Force Secretary Heather Wilson and Army Secretary Mark Esper, an acknowledgement that someone who has already undergone Senate confirmation is considered a safer bet.
Mulvaney said the president "is entitled to have a secretary of defense who is committed to that same end".
The announcement that USA troops would leave the civil war-torn country - where they have been deployed to assist in the multinational fight against the Islamic State in Iraq and Syria (ISIS) group - shocked global partners and American politicians alike.
As Mattis played an important role in embodying Trump's strategic concept and buffering the president's strategic impulse, Zuo said, his departure would make it easier for Trump to further "rip off" his allies.
Just days after Defense Secretary Jim Mattis announced he would step down from that post in late February, Trump announced by Twitter he would push the popular Cabinet member out on January 1.
US Gov't now in partial shutdown
Most of the Internal Revenue Service (IRS) will be sent on unpaid leave, including those who assist taxpayers with queries. The US Postal Service, which is busy in the holiday season, will not be affected because it is an independent agency.
Trump, who tweeted on Thursday that Mattis was "retiring, with distinction, at the end of February", made his displeasure clear on Saturday by tweeting that the retired Marine general had been "ingloriously fired" by former President Barack Obama and he had given Mattis a second chance.
Turkey's President Recep Tyyip Erdogan and President Trump have agreed in a phone conversation to prevent a power vacuum in Syria after American ground forces withdraw.
President Donald Trump, who aides said has been frustrated by news coverage of Defence Secretary James Mattis's scathing resignation letter, abruptly announced Sunday that he was removing Mattis two months before his planned departure and installing Patrick Shanahan as acting defence secretary.
But Trump's announcement appeared to forestall that trip.
Just after announcing his Syria decision, Trump again confounded worldwide partners with plans to slash troop numbers in Afghanistan. "It is the very definition of patriotism and integrity", Tom Mattis added.
Asked whether Trump wanted a Pentagon leader willing to challenge him or someone in lock step with his views, Mulvaney said "a little bit of both".
"Since Washington crossed the DE at Christmas in 1776, American troops have missed holidays at home to defend our experiment in democracy", he says. Let's hope that we see more of that in the New Year.