US Defense Secretary James Mattis listens as US President Donald Trump (not pictured) speaks to the news media while gathering for a briefing from his senior military leaders in the Cabinet Room at the White House in Washington, US, Oct 23, 2018.
Trump announced on Sunday that he would remove Mattis and replace him with his deputy, Patrick Shanahan, instead of allowing a successor to be confirmed and Mattis to conduct a proper transition over a two-month period. He had worked for more than three decades at Boeing Co. and was a senior vice president when he became Pentagon deputy in July 2017.
Yuri Gripas/ReutersDefense Secretary James Mattis.
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.
Obama removed Mattis as head of US Central Command in 2013 because of what officials at the time said were perceived to be his hawkish views on Iran.
While the Defense Secretary is famous for his over 40 year Marine Corps career, Tom Mattis was the first to join the Marines.
In the letter, Mattis made clear that he didn't see eye to eye with a president who has expressed disdain for North Atlantic Treaty Organisation.
The departure of Defence Secretary Jim Mattis in a week will thrust President Donald Trump's largely unknown choice for acting Pentagon chief into the military hot seat and shift attention to the search for a permanent replacement who will probably face sharp Capitol Hill questioning about the administration's murky foreign policy. Mattis had been at work on Friday, and defence officials had insisted he was planning to stay through February, when he would attend a North Atlantic Treaty Organisation defence ministers meeting. Lindsey Graham, R-S.C., who was among those attending.
Mattis went to the White House that day in an effort to get Trump to keep United States forces in the war-torn country.
The exit of Mattis, highly regarded by Republicans and Democrats alike, added to concerns over what many see as Trump's unpredictable, go-it-alone approach to global security. It was that for sure, I think that was the tipping point. "But keep in mind it's not unusual for a president to lose members of the Cabinet over these types of disagreements".
Mr Trump wrote: "I just had a long and productive call with President @RT_Erdogan of Turkey".