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The two Asian countries were chosen as the destinations for the first overseas visit by a Trump Cabinet member and US Defense Secretary James Mattis gave Seoul and Tokyo exactly what they were looking for.
Both countries had been stunned by comments made by President Donald Trump on the campaign trail that suggested that they should develop their own nuclear weapons to defend themselves and pay more toward the cost of troops stationed in the region.
He even told his Japanese counterpart Tomomi Inada that her country had "been a model of cost sharing."
"Mattis said all the right things. His goal was clearly to reassure," said Anthony Ruggiero, senior fellow at the Foundation for Defense of Democracies.
Disputed islands
There'll be fewer smiles in China and North Korea.
His comments quickly drew a rebuke from China, which accused the US of injecting instability into the region.
"We urge the US side to adopt a responsible attitude and stop making wrong remarks on the issue of the sovereignty of Diaoyu Islands. Only doing so would avoid further complicating the issue or injecting an element of instability to the region," said Ministry of Foreign Affairs spokesman Lu Kang.
Tong Zhao, an associate at Carnegie-Tsinghua Center for Global Policy in Beijing, said China would be alarmed at the deepening security relationship between the US and Japan.
"This will be a big disappointment to Beijing," he said.
"We all play by the rules, and if we have disputes we take them to arbitration, we don't settle them by taking military means and occupying land that is subject to question, to say the least, about who actually owns it," he said.
Phew?
Trump has assembled an eclectic Asia team, and it's not yet clear whose views will hold sway.
Mattis' stance, as advanced during his visit, appears to be a continuation of President Barack Obama's policies in the region -- clashing with the more muscular approach towards China coming from Trump and other members of his inner circle.
For example, Mattis said Saturday the solution to the dispute in the South China Sea was diplomatic and didn't see any need at all for any "dramatic military moves" -- a position that appears to put him at odds with Secretary of State Rex Tillerson.
This uncertainty over who has Trump's ear means the relief being felt in Seoul and Tokyo in the wake of Mattis' reassurances is still laced with doubt.
"How much influence Mattis has in actually crafting Asia strategy... remains very much an open question," says Ashley Townshend, a research fellow at United States Studies Centre at the University of Sydney.