"The weapons you are acquiring are not making you safer, they are putting your regime in grave danger," Trump said during an address at South Korea's National Assembly in Seoul. "Every step you take down this dark path increases the peril you face."
In a bruising insult of the repressive regime founded by Kim Il-sung in the middle of last century and governed in his image ever since, Trump diminished the kingdom now ruled by his grandson.
"North Korea is not the paradise your grandfather envisioned," he said. "It is a hell that no person deserves."
It was a frank and cutting personal insult directed at the man who Trump has previously derided as "rocket man." He avoided that term on Wednesday, but was unsparing in his description of a failed state where the majority of citizens live in misery.
And while Trump similarly avoided his prior threats to rain "fire and fury" on North Korea should their nuclear threats persist, he was unequivocal in his vows to counter provocations with military force.
"That would be a fatal miscalculation," he said of North Korean threats to strike the US and its allies. "This a very different administration than the United States has had in the past. Do not underestimate us. And do not try us."
Trump reiterated that point on Twitter Wednesday evening: "NoKo has interpreted America's past restraint as weakness. This would be a fatal miscalculation. Do not underestimate us. AND DO NOT TRY US," he tweeted.
Earlier in the day, Trump attempted a symbolic stare-down of Kim
at the heavily fortified border that separates North and South Korea. But heavy fog stymied his plans, and he was forced to turn back.
According to his aides, the scrapped plans frustrated Trump, who wanted to provide symbolic heft to his warnings to Pyongyang.
'The time for excuses is over'
North Korea's provocations have provided the underpinning for Trump's
intensive talks in Asia at the start of his marathon tour of the continent. He received support in Japan for his combative stance, but in Seoul -- positioned 35 miles from the border with the North -- Trump's fiery rhetoric has been met with unease.
During a series of public appearances in and around the South Korean capital on Tuesday, Trump defended his provocative threats toward Kim, but declined to repeat the type of fiery bombast which has helped ratchet up tensions here. And while he again declined to rule out use of military force in countering North Korea's threats, he appeared less willing to provoke the communist regime than he has from the United States.
Standing before the South Korean legislature, Trump's audience extended beyond the walls of the soaring assembly hall situated on the banks of the Han River in central Seoul. He spoke to American listeners, and to leaders in Moscow and Beijing, as he exhorted the international community to step up its attempts to isolate North Korea.
"The time for excuses is over. Now is the time for strength. If you want peace, you must stand strong at all times," Trump said. "The world cannot tolerate the menace of a rogue regime that threatens with nuclear devastation."
Trump singled out China, where is heading immediately after the speech, and Russia, whose president he will meet later this week.
"We call on every nation -- including China and Russia -- to fully implement UN Security Council resolutions, downgrade diplomatic relations with the regime and sever all ties of trade and technology," Trump said.
"It is our responsibility and our duty to confront this danger together," Trump said. "Because the longer we wait the greater the danger grows and the fewer the options become."
At no other point on his 13-day tour of Asia will Trump have a similar opportunity to lay out at length his plan to help secure American allies while also pursuing aggressive trade policies
he believes will put the United States on fairer footing.
Drafts of the speech were in the works for weeks, officials say, with input from Trump's top national security aides like Secretary of State Rex Tillerson and national security adviser HR McMaster. Trump's speechwriter Stephen Miller is accompanying the President in Asia.
The address drew heavily upon the long history of American military support for South Korea, with references to the Korean War-era Inchon landings and the Battle of Pork Chop Hill that featured tens of thousands of American servicemen flooding the peninsula to counter the advance of communism on the peninsula.
Trump used the economic successes of South Korea in the decades since that war to further deride the hermit nation to the north.
"North Korea is a country ruled by a cult. At the center of this military cult is a deranged belief in the ruler's destiny to rule as a parent protector over a conquered Korean Peninsula," Trump said. "The more successful South Korea becomes the more successfully you discredit the dark fantasy at the heart of the North Korean regime."
Meanwhile, Pyongyang was closely watching Trump's key speech, according to North Korean officials authorized to speak to CNN on behalf of the regime.
The officials told CNN's Will Ripley ahead of the address that North Korea is not yet interested in talks with the United States despite Trump's conciliatory tone in South Korea. US officials point out three American citizens are currently in North Korean custody, and any diplomacy would also need to involve discussions for their release.
According to officials in North Korea and the United States, diplomatic channels are still closed after Trump's fiery UN speech in September. However, North Korea won't rule out future talks, but still feels the need to prove their nuclear capabilities, which means more tests, the officials said.
South Koreans gathered to mark Trump's address, with local police protectively lining the streets.
CNN's Paula Hancocks reported: "More Trump supporters than protestors today. Some traveled from other side of the country to welcome him."