Speaking to cheering servicemen at Yokota Air Base just west of Tokyo, Trump donned a military jacket offered to him and issued a threat that "no one, no dictator, no regime and no nation should underestimate... American resolve".
"Every once in a while in the past, they underestimated us. It was not pleasant for them, was it?" roared Trump.
"We will never yield, never waver and never falter in defence of our people, our freedom and our great American flag."
The president's first stops are Japan and South Korea -- frontline US allies in the effort to force Pyongyang to abandon its nuclear programme, and the two countries with most to fear should a full-scale conflict break out.
More From This Section
Trump touched down under clear blue Tokyo skies and stepped out with his wife Melania in bright sunshine to greet the crowds.
Speaking to reporters on the plane, he announced he would likely be meeting Russian President Vladimir Putin during the tour, as the international community battles for a solution to the North Korean missile crisis.
He added that North Korea was a "big problem for our country and for the world, and we want to get it solved" but had kind words for the people in the hermit state.
"I think they're great people. They're industrious. They're warm, much warmer than the world really knows or understands. They're great people. And I hope it all works out for everybody," he said.
The next stop for Trump is a golfing date with his "friend" Shinzo Abe, prime minister of Japan, which has seen missiles fired over its northern island amid threats by Pyongyang to "sink" it into the sea.
The Japanese leader has emerged strengthened from a crushing victory in a snap election and has firmly supported Trump in his policy of exerting maximum pressure on Kim, backed up with the threat of military force.
"I want to further cement the bond of the Japan-US alliance, based on our relations of trust and friendship with President Trump," said Abe as Trump arrived.
Trump for his part described Japan as a "treasured partner and crucial ally of the US".
While Trump has been in regular contact with the hawkish Abe during the North Korean missile crisis, he pointedly failed to speak to South Korean President Moon Jae-in for several days after Pyongyang's second intercontinental ballistic missile test in July.
Analysts point to Abe and Moon's contrasting approaches to the crisis as an underlying factor, although both leaders will be hoping to press Trump into reaffirming Washington's steadfast commitment to their defence.
"The two sides have subtle differences in their positions," said Kim Hyun-Wook, a professor at the Korea National Diplomatic Academy. Trump will not follow the well- trodden path to the De-Militarized Zone dividing the Korean peninsula -- a visit derided in Washington as a bit of a "cliche."
From Seoul, Trump travels to China to meet his counterpart Xi Jinping who, like Abe, has solidified his grip on power, after being handed a second term.