President Donald Trump’s decision Thursday to cancel his summit with North Korea’s Kim Jong Un was more than just a blow to peace in the Korean Peninsula.

It made Hawaii more vulnerable to a missile attack, says an expert on missile defense systems.

Riki Ellison, chairman of the nonprofit Missile Defense Advocacy Alliance, called for the U.S. to work immediately to shore up missile defenses in the islands in the wake of Thursday’s diplomatic setback.

The frightening prospect of North Korean nuclear missiles being lobbed at the islands lessened substantially after news of the summit emerged in March.

“Today that honeymoon looks to be over,” Ellison said.

Trump’s letter to Kim not only abruptly cut off the meeting, but turned up the volume on belligerent rhetoric. “You talk about your nuclear capabilities,” Trump wrote, “but ours are so massive and powerful that I pray to God they will never have to be used.”

Members of Hawaii’s congressional delegation joined other Democrats on Thursday in blasting the president’s move.

“We should listen to career diplomats, nonproliferation professionals, and experts in North Korea. He is not good at this.” U.S. Sen. Brian Schatz tweeted.

U.S. Rep. Tulsi Gabbard tweeted, “The seriousness of North Korea’s nuclear threat to the American people cannot be overstated. The need for direct talks between Trump and Kim is critical and urgent to peacefully negotiate denuclearization of #NorthKorea, remove nuclear threat, and avoid all-out catastrophic war.”

Ellison said he could easily see an escalation of hostilities if and when North Korea resumes testing of intercontinental ballistic missiles and hydrogen bombs. Trump might respond with a strike on a missile facility, he said, while Kim answers with a launch on U.S. territory.

Guam is closer but Hawaii has fewer missile defenses, relying substantially on the 40 American missile interceptors based in Alaska.

While the U.S. military insists they are enough to protect the islands, Ellison said Hawaii is at risk because those interceptors are likely to be given priority to protect the mainland if it is also under attack.

Additionally, he said, Hawaii is so far away from Alaska that there will be no second chances if an interceptor misses.

“Hawaii is defended, but not as much as the other 49 states,” he said.

North Korea has longer-range missiles capable of hitting the U.S. mainland, but the more accurate ones are the short-range missiles, Ellison said.

Hawaii needs a multilayered missile defense, Ellison said, and the most expedient and least expensive path is to install a ground-based Aegis ballistic missile defense site at the Pacific Missile Range Facility on Kauai. One option is to install a missile at the same site on the base where one was tested in February.

The U.S. could also deploy a Terminal High-Altitude Area Defense battery in Hawaii as it did briefly in 2009, he said.

Denny Roy, an Asia-Pacific security issues expert with the East-West Center, said the future of the nuclear standoff remains unclear following Trump’s action Thursday.

It could mark the end of the 2018 thaw and a return to the missile testing and bellicose exchanges of 2017, Roy said, or it could be a temporary setback if the summit is rescheduled following a cooling-off period.

“It doesn’t benefit Hawaii in any way,” he said.

Roy said he disagrees that Hawaii is any more vulnerable than it was before talk of North Korean denuclearization and Nobel Peace Prizes emerged following news of the summit.

Roy said if North Korea would fire on a populated area, the regime would certainly end in short order, and such an action would occur only as an extreme and desperate choice. Tokyo, Seoul or a U.S. East Coast city would be a more likely large-population target, he said.

The North Korea threat was ramped up in Hawaii when the state resurrected its ballistic missile attack warning siren testing in December. After January’s false missile attack, the threat escalated in many people’s minds, although testing was eventually discontinued.

Plans for a summit between Trump and Kim offered a welcome respite to the tensions of the peninsula, Roy said, even though the possibility of ultimate success was small.