President Donald Trump called the collapse of a planned summit with Kim Jong Un a setback for both North Korea and the world, and said the U.S. military is ready if necessary in the event of a conflict on the Korean peninsula.

“While many things can happen and a great opportunity lies ahead, potentially, I believe this is a tremendous setback for North Korea and indeed a setback for the world,” Trump said at the White House hours after releasing a letter to Kim canceling the meeting.

Trump said he had spoken with Defense Secretary Jim Mattis and the leaders of South Korea and Japan. The U.S. military is “ready if necessary,” he said, and the two Asian allies “are not only ready should foolish or reckless acts be taken by North Korea, but they are willing to shoulder much of the financial cost or burden” of a conflict.

But Trump also held out hope that the June 12 summit in Singapore could get back on track, or that he and Kim could meet in the future. “Nobody should be anxious. We have to get it right,” he said.

South Korean President Moon Jae-In said that peace on the peninsula shouldn’t be abandoned and suggested that Trump and Kim hurt chances for a successful summit by speaking to each other through statements, tweets and spokespeople.

“It’s hard to resolve the diplomatic issue, which is both difficult and sensitive, with current way of communication,” Moon said in a statement. “I wish the leaders would have a more direct and closer conversation to deal with it.

Trump sounded a positive note as he left a bill signing event, telling reporters “the dialogue was good until recently” with Kim. And “Kim Jong Un wants to do what’s right,” adding, “I really believe that.”

“It’s only recently that this has been taking place and I think I understand why it’s been taking place,” he said. He declined to explain further, but Trump said earlier this week that planning for the summit had been proceeding well until Kim met May 8 with his closest ally, Chinese President Xi Jinping, who is negotiating a trade dispute with Trump.

China wasn’t surprised by the collapse of the summit given recent signals that had come from Trump, a government official said. The official asked not to be identified commenting on the matter.

‘Open Hostility’

In his letter to Kim pulling out of the summit, Trump cited “tremendous anger and open hostility” in recent statements from Pyongyang.

North Korea hardened its rhetoric toward the U.S. earlier Thursday, lashing out after remarks by Vice President Mike Pence and the White House national security adviser, John Bolton, that had linked the country with Libya. Choe Son Hui, vice-minister of foreign affairs, called Pence a “political dummy” and his comments “unbridled and impudent,” according to an English-language statement from North Korea’s state-run KCNA.

Choe warned her nation was prepared for a “nuclear-to-nuclear” showdown if the U.S. didn’t follow through on the summit. “We can also make the U.S. taste an appalling tragedy it has neither experienced nor even imagined up to now,” she said, warning that she would recommend Kim cancel the summit if U.S. officials didn’t curb their language.

Trump beat Kim to it, issuing his own threat. “You talk about your nuclear capabilities, but ours are so massive and powerful that I pray to God they will never have to be used,” Trump wrote.

Stocks declined on the news, with the S&P 500 index down 0.3 percent at 12:28 p.m. New York time.

Pyongyang Silence

With the meeting abandoned — at least temporarily — the next steps are unclear. Trump has said the U.S. would continue exerting maximum economic pressure on Kim and his regime.

Secretary of State Mike Pompeo told the Senate Foreign Relations Committee shortly after the letter was released that he was “still optimistic” the U.S. and North Korea would reach a historic deal.

Pompeo said the U.S. had tried in recent days to put teams together to prepare for the meeting and “we had received no response to our inquiries from” the North Korean government.

South Korean President Moon Jae-in called top aides to an emergency meeting late Thursday in Seoul to discuss the developments. Kim Eui-kyeom, a spokesman for Moon, said in a text message that his government is still “trying to figure out” Trump’s intentions.

“We can expect North Korea will condemn the decision in strong terms and cast blame on the United States for throwing away a good thing through its actions,” said Mintaro Oba, a former U.S. State Department official who worked on North Korean issues. “That does raise concerns that Trump will respond in a way that further escalates tension to ‘fire and fury’ levels and beyond.”

Denuclearization Stumble

The highly anticipated summit had been cast by the White House as an opportunity to stave off a military conflict with North Korea and showcase Trump’s ability to make progress where his predecessors had struggled. The president has openly entertained the idea that he could have been awarded the Nobel Peace Prize had the meeting led to a peace agreement between North Korea, the U.S. and South Korea. The countries are technically still at war.

But Trump ultimately ran into the same diplomatic quandary that has flummoxed U.S. presidents for the past 25 years: the inability to persuade a stubborn regime to give up a nuclear program that it regards as key to its survival.

Richard Haas, president of the Council on Foreign Relations and a former senior State Department official under President George W. Bush, said in a tweet that the summit “was bound to fail” because the Trump administration “badly overestimated what NK would agree to; the issue was/is US willingness to accept an outcome short of total denuclearization.”

“All or nothing foreign policy w NK, Iran, China trade risks producing nothing or conflict,” Haas added.