The White House’s surprise announcement Thursday that President Trump had agreed to meet  North Korean leader Kim Jong Un “at a place and time to be determined” has upended expectations about how disarmament talks with the regime would unfold.

Nuclear disarmament advocates, who have long encouraged direct talks between the U.S. government and North Korea, praised the president’s decision while raising questions about the details.

William J. Perry led talks with North Korea in the late 1990s after serving as President Bill Clinton’s secretary of defense and has since emerged as one of the United States’ chief advocates for nuclear disarmament.

Perry described the move as “very encouraging” and “a major improvement over diplomacy that consisted of shouting insults at each other.” But he warned that talks should be based on realistic expectations of what can be negotiated and what can be verified. Perry added:

Skeptics have warned that Washington shouldn’t give Kim the legitimacy of a grand summit with the U.S. president — something he and his forebears have long sought — without North Korea giving up something significant. Already, the country has agreed to suspend missile and nuclear tests as the talks proceed.

Abraham M. Denmark, a top Pentagon official working on Asia policy during President Barack Obama’s administration, said the United States could demand certain actions before organizing the summit.

Jeffrey Lewis, an arms control advocate at the Middlebury Institute of International Studies at Monterey, raised similar concerns.

Former U.N. ambassador John Bolton, a Republican hawk who recently outlined his case for conducting a military strike on North Korea, said in an interview with the Washington radio station WMAL that Trump’s move was “definitely diplomatic shock and awe.”

Bolton said Trump had short-circuited North Korea’s plan of obtaining the capability to strike the United States with a nuclear weapon and then stretching out negotiations for months that could distract the American government before making an official announcement about achieving the capability. Bolton envisioned the meeting between Trump and Kim as an opportunity to deliver a threat of military action:

“I think this session between the two leaders could well be a fairly brief session where Trump says, ‘Tell me you have begun total denuclearization, because we’re not going to have protracted negotiations, you can tell me right now or we’ll start thinking of something else.’ ”

Sen. Lindsey O. Graham (R-S.C.) released a statement on North Korea that suggested the United States would go to war with the country if Kim “tries to play” Trump in any negotiations. He said:

Many say the jury is out on whether the meeting is a good idea, though it has positive prospects. They say it all depends on how U.S. officials manage the process and figure out what exact concessions they are going to receive in what sort of document and with what kind of follow-up. Suzanne DiMaggio, a senior fellow at the New America think tank, who has led outreach to North Korea among think tank representatives, said:

Former George W. Bush administration official Victor Cha, who was the Trump administration’s choice to serve as ambassador to South Korea before his withdrawal, said North Korea has agreed to freeze testing, permit U.S.-South Korea military exercises to proceed and meet with the U.S. president. Cha said the big question is what exactly the United States plans to concede in return.

Many of Trump’s supporters, meanwhile, presented the meeting as a breakthrough and reveled in the tempered praise of Democrats who have long advocated for direct talks with North Korea and an approach to the country that doesn’t involve a preemptive strike. Former Arkansas governor Mike Huckabee, whose daughter Sarah Huckabee Sanders serves as White House press secretary, feigned a Twitter heart attack:

Basketball star Dennis Rodman, who has visited North Korea, cheered Trump’s move. Rodman told The Washington Post that Trump was “on the way to a historical meeting no U.S. president has ever done” and said he looked forward to bringing “more basketball diplomacy to North Korea in the upcoming months.” He asked Trump to send his regards to Kim and his family.

Read more: U.S.-South Korea military exercises are on, despite Trump’s planned meeting with Kim. Here’s what will happen.