Mattis, who is a retired Marine general, resigned his post December 20 in an apparent dispute with President Trump on the decision to withdraw U.S. forces from Syria.
Last week, Trump ordered the withdrawal of USA troops in Syria, a decision that led to the resignation of Defense Secretary Jim Mattis.
In a series of tweets, Trump appeared to question why he had put Mattis in his Cabinet in the first place and said Deputy Defense Secretary Patrick Shanahan will take over as acting secretary on January 1 to cover the accelerated departure.
On Christmas Eve, Mattis sent an official memo to all American service members - possibly his last official memo as secretary of defense.
The video is Mattis' final holiday message to the troops as Defense Secretary.
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Tom Mattis also says he stands by his brother's decision to resign.
In the resignation letter, James Mattis said he planned on staying until the end of February to ensure a permeant replacement could be chosen and to attend a previously scheduled North Atlantic Treaty Organisation meeting.
According to Task & Purpose, the video was recorded on December 19 - one day before Mattis announced his resignation. "These are two sides that are fighting against one another, and neither one of them will achieve a military victory at this stage". It is hard work, but this is nothing new: "Since Washington crossed the DE on Christmas Day in 1776, American troops have missed holidays to defend our citizens' experiment in democracy". "In this world awash with change, you hold the line". "Most don't know your names but all are confident their freedoms and families will be kept safe". "Far from home, you have earned the gratitude and respect of your fellow citizens and it remains my great privilege to serve alongside you".
He does not elaborate on what storm clouds he may see.