President Trump delivered a warning to North Korean leader Kim Jong Un not to underestimate U.S. resolve in the face of Pyongyang's nuclear threats. Trump also challenged other nations — especially China — not to stand idly by while Kim pursues his aggressive nuclear program.

"The time for excuses is over — now is the time for strength," Trump declared in a speech Wednesday morning to South Korea's National Assembly. "The world cannot tolerate the menace of a rogue regime that threatens it with nuclear devastation. All responsible nations must join forces to isolate the brutal regime of North Korea."

The president tried to add an exclamation point to the message, attempting a surprise visit before the speech to the Demilitarized Zone dividing North and South Korea. Trump's helicopter took off from Yongsan Garrison in Seoul, but heavy fog prevented it from landing in the DMZ, and he was forced to turn back after about 20 minutes.

White House spokeswoman Sarah Huckabee Sanders said Trump was frustrated not to make it to the DMZ for what she said would have been a "historic" joint appearance there with South Korean President Moon Jae-in. Sanders added that just making the effort shows the importance to Trump of the U.S. alliance with South Korea.

In his address to the National Assembly, Trump described the roots of that alliance in the Korean War that divided the peninsula, and the remarkable flowering of South Korea's economy in the six decades since.

"On this peninsula, we have watched the results of a tragic experiment in a laboratory of history," Trump said. "The results of this experiment are in and they are totally conclusive."

Trump argued that the success of South Korea stands as a rebuke to the communist North and must be defended against Pyongyang's aggression.

"America does not seek conflict or confrontation, but we will never run from it," Trump said. "We will not be intimidated. And we will not let the worst atrocities in history be repeated here, on this ground we fought and died so hard to secure."

To underscore that message, the president noted that American aircraft carriers and nuclear submarines have been stationed in waters not far from the Korean Peninsula.

"The regime has interpreted America's past restraint as weakness," Trump said. "This would be a fatal miscalculation. Today I hope I speak not only for our countries but for all civilized nations when I say to the North, 'Do not underestimate us. And do not try us.' We will defend our common security, our shared prosperity, and our sacred liberty."

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