By Agence France-Presse
It's now a familiar role for the Pentagon chief, not just in Asia but across Europe and the Middle East. With Trump disrupting one relationship after another, provoking rivals and unnerving friends, Mattis is the one making sure they can still work on their traditional foundations.
This week, he had to tell the leaders of South Korea and Japan -- close allies who have depended on the US security umbrella for decades -- that Washington remains committed to protecting them.
That was always understood before Trump suddenly canceled defense exercises with South Korea as an enticement to North Korea's Kim Jong Un to negotiate giving up his nuclear weapons.
And Mattis had to cut through the noise of Trump's trade war threat against China to deliver one of the toughest warnings in recent years, that the US has grown dangerously impatient with Beijing's military expansionism in the region.
The challenge was to get the Chinese to focus on his terse, carefully calibrated message and not conflate the US president's trade hoopla, so that they do not overreact.
"I'm here to keep our relationship on the right trajectory, keep it going in the right direction and to share ideas with your military leadership, as well as look at the way ahead," the Pentagon chief told Chinese President Xi Jinping.
Mattis, a 67-year-old Marine veteran of multiple wars and conflicts, and a deep student of history, insists he is only advancing administration policy.
But under Trump, it is an uphill battle to prevent key relationships from souring.
Japan and South Korea alike have been deeply worried about the US security commitment after Trump's historic summit with Kim in Singapore on June 12.
As they agreed to enter into denuclearization talks, Trump abruptly canceled a key US-South Korea exercise slated for later this year.
Despite their longstanding effect of keeping North Korea at bay, Trump called the war games "expensive" and "provocative."
How worried were the allies? In Seoul, Mattis had to declare bluntly that US troop levels in South Korea, the central pillar of the US defense commitment, would not change, and reiterate that the US commitment was "ironclad."
In Tokyo, he pledged a continuing strong, "collaborative" defense stance with regional allies, and that Japan will remain a "cornerstone" of regional stability.
His predecessors never had to travel to the region to make that so clear.
And yet, with US negotiations with Pyongyang hyper-secret, allies still aren't sure.
"They are increasingly concerned and worried about the reliability of our reassurances," said James Schoff, a former senior Pentagon East Asia specialist now in the Carnegie Asia Program.
In meetings Friday with Prime Minister Shinzo Abe and Japan's foreign and defense ministers, Mattis was repeatedly reminded that if the US negotiates only to eliminate North Korea's nuclear weapons and long-range ballistic missiles which can strike the United States, Japan will still feel deeply vulnerable.
Tokyo pressed the US defense chief to ensure that North Korea's other weapons of mass destruction, chemical and biological weapons, and its short- and medium-range ballistic missiles that can deliver them to South Korea and Japan, are also included.
Mattis could not make that promise, saying the talks are in the hands of "the diplomats."
But he said the two sides discussed the possibility of deepening their military relationship, and told Japanese Defense Minister Itsunori Onodera "to remember you have a friend in the Pentagon."
In China, Mattis sought to warn that Beijing weaponizing small islands in the South China Sea and its stepped-up pressure against Taiwan, the longtime US security ally that China views as a renegade province, were close to going too far.
It was possibly one of the most serious and delicate messages Mattis had to deliver yet in the 17 months of the Trump administration.
But it came as Trump appeared on the verge of launching an all-out commercial war against Beijing, creating the possibility that China would conflate the two and overreact.
It was a tense balancing act, and though US officials said their Chinese opposites were positively engaged in the discussions, in a statement Xi toughly rebuffed the criticisms on the South China Sea and Taiwan, saying China would not give up "one inch" of its territory.
But the reception Mattis had in Beijing, meeting not only Xi but the country's top military leaders, showed they were ready to pay attention.
Still, the question leaders in East Asia and elsewhere have is, as trustworthy as Mattis is, does Trump pay him any heed?
"There are some things the Asian leaders can't control. Mattis clearly realizes there are some elements that he may not be able to control," said Schoff.