President Donald Trump praised Japan as a "crucial ally" and warned adversaries not to test America's resolve as he kicked off his first trip to Asia.
Mr Trump landed at Yokota Air Base on the outskirts of Tokyo and was greeted by cheering service members.
He then donned a bomber jacket for a speech in which he touted American firepower and the US alliance with Japan.
"Japan is a treasured partner and crucial ally of the United States and today we thank them for welcoming us and for decades of wonderful friendship between our two nations," he said, speaking in front of an American flag inside an airplane hangar.
Mr Trump was expected to spend much of his 12-day, five-country Asian tour exhorting allies and rivals to step up efforts to counter the dangers posed by North Korea, which continues to move forward with its nuclear weapons program.
Both he and North Korean leader Kim Jong Un have been engaged in an escalating war of words, with Mr Trump repeatedly referring to the North Korean leader as "Little Rocket Man" and threatening in a recent speech to "totally destroy" the nation if necessary.
Even before he landed, Mr Trump used the first moments of the trip to denounce the North as "a big problem" that must "be solved."
"There's been 25 years of total weakness, so we are taking a very much different approach" toward the North, he told reporters travelling with him.
During the speech, Trump did not mention North Korea by name, but warned of the consequences of crossing what he called the "most fearsome fighting force in the history of our world".
"Together with our allies, America's warriors are prepared to defend our nation using the full range of our unmatched capabilities. No one - no dictator, no regime and no nation - should underestimate, ever, American resolve," Mr Trump told the troops.
Some regional analysts have speculated that Trump's presence in Asia may prompt North Korea to take provocative action, such as another missile test.
The president, when asked about that possibility aboard Air Force One, said "we'll soon find out."
After the speech, Mr Trump flew by helicopter to the Kasumigaseki Country Club about 20 miles outside of Tokyo for lunch and round of golf with Japan's Prime Minister Shinzo Abe and Japanese pro golfer Hideki Matsuyama.
At the club, the leaders signed white caps inscribed with the phrase, "Donald and Shinzo: Make Alliance Even Greater," a tribute to the US-Japan friendship and a play on Trump's campaign slogan.
The White House has signalled that Mr Trump will push American economic interests in the region, but the North Korea issue is expected to dominate the trip.
He will forego a trip to the Demilitarised Zone, the stark border between North and South Korea.
All US presidents except one since Ronald Reagan have visited the DMZ in a sign of solidarity with Seoul.
The White House contends that Mr Trump's commitment to South Korea is already crystal clear, as evidenced by his war of words with Kim and his threats to deliver "fire and fury" to North Korea if it does not stop threatening American allies.