He also acted to stanch speculation that the United States, as White House officials suggested, might act precipitously against perceived threats from China and Iran, saying that military steps were not required. This week, Mattis spoke to Mexican defence leaders, highlighting cooperation in the wake of Trump's high-profile feud with President Enrqiue Peña Nieto.
Derek Chollet, who was a senior Pentagon official under President Barack Obama, said that allies were monitoring Mattis's statements for clues about whether the new Administration would follow a course set by Trump's campaign statements, or stay broadly within the borders of established US foreign policy. "Trump may tweet up a storm, but if there is little or no connectivity to what happens on the ground, they may start discounting it," he said.
While he has been held up by Trump critics as a bulwark against the President's whims and praised by supporters for his military record, it's not yet clear as the rest of Trump's Cabinet moves into place what sway Mattis will ultimately hold in shaping major decisions.
In addition, the role of quiet diplomat is an unlikely one for a longtime combat commander whose brash commentary has occasionally generated controversy.
But Mattis, who has already shown himself willing to disagree with the President's preferences, now occupies a key position in the Cabinet of a man with little foreign policy experience.
Unlike Trump and some of his White House advisers, including Stephen Bannon and son-in-law Jared Kushner, Mattis has worked within the US military and security establishment for virtually his entire career. Although he appears to share the alarm that senior White House officials see in potential threats from Iran's missile programme and North Korea's nuclear ambitions, his path has been shaped by different forces.
His affinity for working with allies is a product of his experience in the Nato mission in Afghanistan and the first Gulf War. As head of US Central Command, he conferred closely with Arab nations about terrorism and Iran's actions in the region, and oversaw the US military's exit from Iraq in 2011.
His views on Iran were shaped by the 1983 bombing of the Marine barracks in Lebanon. Mattis's hawkish approach to Tehran eventually alienated him from some in the Obama White House before he left CENTCOM in 2013.
Trump, who has surrounded himself with former generals, has already shown that he is willing to defer to Mattis on issues such as whether the United States should employ waterboarding on detainees. In his confirmation hearing, he suggested a less friendly attitude toward Russia than the President has espoused and stressed the importance of Nato, despite Trump's questions about the alliance's relevance.
"Secretary Mattis has found a way to reaffirm alliances without disagreeing explicitly with his commander-in-chief," said Michael O'Hanlon, a scholar at the Brookings Institution. "That is enormously important."
The President's apparent support for Mattis's military judgment may enhance the new secretary's standing in internal discussions or with allies, potentially putting the Pentagon boss in a position similar to that of former Defence Secretary Robert Gates, who wielded significant influence in policy debates under Obama, sometimes to the frustration of the White House, Chollet said.
I don't think he hired mice. He hired people with bold backgrounds who aren't afraid to stand up to him
His power could be enhanced if he and Secretary of State Rex Tillerson develop a relationship allowing them to jointly advocate policy positions, like Gates frequently did with former Secretary of State Hillary Clinton.
But already Mattis, like other senior officials, has appeared to have been on the outside of some White House decision-making in the Administration's first weeks. He was widely reported to have received little notice that Trump, in his first visit to the Pentagon, on January 28, would sign an executive order barring the entry of migrants from certain majority-Muslim nations, including Iraq.
Pentagon officials subsequently pushed to clarify that Iraqis who obtained special visas after working with the US Government would be admitted.
In other areas, Mattis has used his access to the President to secure approval for actions put forward by the military, notably the deadly Special Operations raid in Yemen that occurred a week after Trump took office.
JV Venable, a senior fellow at the Heritage Foundation, said that Trump is likely to give Mattis "strategic direction" and then allow him greater rein, despite potential disagreement about particular decisions, than recent defence secretaries to manage military matters.
"I don't think he hired mice," Venable said. "He hired people with bold backgrounds who aren't afraid to stand up to him."
