"No one, no dictator, no regime... should underestimate American resolve," Trump told cheering servicemen at the Yokota Air Base on his arrival in Japan.
Trump had earlier described Japan as a "treasured partner and crucial ally of the US", as Asian partners look for reassurance that Washington will back them if conflict with North Korea were to break out.
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The president was speaking at the start of a marathon Asian tour that will take in five countries, starting with a trip to Japan to visit his "friend" Shinzo Abe.
Before touching down, Trump had announced to reporters on Air Force One that he was expecting to meet his Russian counterpart Vladimir Putin during the trip, as the international community strives for a solution to the North Korea crisis.
"I think it's expected we'll meet with Putin, yeah. We want Putin's help on North Korea, and we'll be meeting with a lot of different leaders," said Trump.
