Now, with the Deputy Secretary of Defense, Patrick Shanahan, bumping up to the top spot, the question remains: who will become Mattis' permanent replacement?
"Deputy Secretary will continue to serve as directed by the president, and the Department of Defence will remain focused on the defence of the nation", Buccino said on Sunday.
The phone call Tom Mattis got from Jim Mattis on Sunday morning wasn't a pleasant one, but he said his younger brother was "unruffled" by President Donald Trump's decision to force him out early, the elder Mattis told The Seattle Times.
Instead, Shanahan will immediately take the reins and Shanahan "could be there for a long time" in the acting defense secretary position, Trump said, according to Reuters, during his surprise visit to Iraq yesterday.
In typical fashion, Trump said Saturday that the media was treating him unfairly over the Syria withdrawal decision.
However, he changed his tone two days later, writing on Twitter that he had given Mattis a "second chance" after he was "ingloriously" fired by then-US president Barack Obama, and appearing to take aim at a line from Mattis' resignation letter about respecting allies.
"I've encouraged him to find people who have some overlap with him but don't see the world in lockstep with him", Mulvaney said.
Andrew Harrer-Pool Getty Images Secretary of State Mike Pompeo Mattis and national-security adviser John Bolton
On Dec. 19, Secretary of Defense James Mattis resigned. Secretary of State Mike Pompeo, not the president, notified Mattis of Trump's decision to put in place Shanahan, said a senior administration official who insisted on anonymity to discuss personnel issues.
A number of names have been floated in recent months, as rumors of Mattis' impending departure have been circulating since well before Trump referred to him as "sort of a democrat" in a recent interview.
Trump's sudden move to push Mattis out was reportedly a retaliatory measure, but Mattis evinced no ire over it when he told his older brother on Sunday. Brett McGurk, the USA envoy to the global coalition combating the Islamic State in Iraq and Syria, resigned shortly after Mattis' decision.
French President Emmanuel Macron said on Sunday he deeply regretted Trump's decision to withdraw USA troops from Syria.
In his letter, Mattis had said he would step down at the end of February to allow for a successor to be confirmed and attend Congressional hearings and a key North Atlantic Treaty Organisation meeting.
Meanwhile, Senate Majority Leader Mitch McConnell, R-Ky., joined leading Republicans on foreign affairs in urging Trump to reconsider his decision to withdraw American forces from Syria.
Turkey was a rare ally that lauded Mr Trump's momentous call on Syria, a country where it will now have a freer rein to target US-allied Kurdish fighters who have played a key role in the anti-ISIS war but are deemed terrorists by Ankara. "After many years they are coming home".
Use Christmas to embrace Jesus Christ - CACI
On this occasion, I join my Christian brethren and sisters to pray for peace and harmony in the world. Mrs Lydia Yusuf said she was happy to celebrate the season in good health.