SEOUL, KOREA, REPUBLIC OF—U.S. President Donald Trump delivered a sharp warning to North Korean leader Kim Jong Un on Wednesday, telling him the weapons he’s acquiring “are not making you safer. They are putting your regime in grave danger.”

In a speech delivered hours after he aborted a visit to the heavily fortified Korean demilitarized zone due to bad weather, Trump called on all nations to join forces “to isolate the brutal regime of North Korea — to deny it any form of support, supply, or acceptance.”

“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,” he told South Korean lawmakers. “We will defend our common security, our shared prosperity, and our sacred liberty.”

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Trump had been scheduled to make the unannounced early morning trip to the DMZ amid heightened tensions with North Korea over its nuclear program.

The Marine One presidential helicopter left Seoul at daybreak and flew most of the way to the DMZ but was forced to turn back just five minutes out due to poor weather conditions. Reporters travelling in a separate helicopter as part of the president’s envoy saw fog through the windows, and weather reports from near the heavily fortified border showed misting conditions and visibility below one mile. Pilots, officials said, could not see the other helicopters in the air.

White House press secretary Sarah Huckabee Sanders said the president was disappointed he couldn’t make the trip. “I think he’s pretty frustrated,” she told reporters. “It was obviously something he wanted to do.”

Before he left for Asia, a White House official had ruled out a DMZ visit for Trump, claiming the president didn’t have time on his schedule and that DMZ visits have become a little cliché.

But Sanders said the visit had been planned well before Trump’s departure for Asia. The trip was kept secret for security reasons, she said.

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Trump had been scheduled to make the visit with South Korean President Moon Jae-in, who travelled separately and landed about a 20-minute drive from the DMZ. Sanders said the military and the U.S. Secret Service had deemed that landing would not be safe, and Trump deferred to them.

After returning to Seoul, administration officials had hoped they might be able to wait out the bad weather and make a second landing attempt. At the U.S. Army’s Yongsan Garrison landing zone, White House Chief of Staff John Kelly and Sanders frequently glanced up at the clouds to see if the sky was clearing. But time would not allow it.

The aborted visit came hours before Trump addressed the South Korean National Assembly before closing out his two-day visit to the nation and heading to his next stop, Beijing.

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Visiting the border that has separated the North and South for 64 years has become something of a ritual for U.S. presidents trying to demonstrate their resolve against North Korea’s ever-escalating aggression. Every American president since Ronald Reagan, save for George H.W. Bush, has made the trip, peering across the barren north through binoculars, hearing broadcast propaganda and reaffirming their commitment to standing with the South.

The attempted visit was scheduled for a day after Trump made a striking shift in tone for a president, who for months has issued increasingly dire threats to answer any hostile North Korean action with “fire and fury.” In a recent speech at the United Nations, Trump said he would “totally destroy” the nation, if necessary, and has derided Kim as “little Rocket Man.”

But on Tuesday, his first day on the Korean Peninsula, Trump signalled a willingness to negotiate as he urged Pyongyang to “come to the table” and “make a deal.” He also he’d seen “a lot of progress” in dealing with Pyongyang, though he stopped short of saying whether he wanted direct diplomatic talks.

“It makes sense for North Korea to come to the table and make a deal that is good for the people of North Korea and for the world,” Trump said at a news conference with Moon. “I do see certain movement.” He also sounded an optimistic note on disagreements with the North, saying confidently, if vaguely: “Ultimately, it’ll all work out.”

Ever the showman, Trump had teased that he had a surprise in store, saying at a Tuesday evening banquet that he had an “exciting day” planned Wednesday — “for many reasons that people will find out.” He did not elaborate on what turned out to be the aborted trip to the DMZ.

North Korea has fired more than a dozen missiles this year but none in nearly two months. Analysts caution against reading too much into the pause.

There’s no public sign of any diplomatic progress between Washington and Pyongyang. U.S. officials say the back channel between the State Department and the North Korean mission at the United Nations in New York remains intact, but contacts have not been substantive other than achieving the release of American college student Otto Warmbier in June. He died days after his repatriation to the U.S.

Still, Trump’s conciliatory comments would be welcome in South Korea, where both the government and the wider population have been unnerved by the president’s threats against the North.

Trump did note the United States’ military options, mentioning that three aircraft carrier groups and a nuclear submarine had been deployed to the region. But he added that “we hope to God we never have to use” the arsenal. And he accused Kim of “threatening millions and millions of lives, so needlessly.”

Moon, who has been eager to solidify a friendship with Trump, said he hoped the president’s visit would be a turning point in the standoff with North Korea.

Visiting the wooded, craggy terrain inside the DMZ is like going back in time to 1953. In July of that year, the Korean War armistice agreement was signed at Panmunjom, the so-called “truce village” bisected by a marker that is the official dividing line between the North and South.

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