A U.S. F-22 Raptor stealth fighter jet lands as South Korea and the United States conduct the Max Thunder joint military exercise at an air base in Gwangju, South Korea. (Park Chul-Hong/AP)

As the Trump administration races to prepare for a possible nuclear summit, a central question looms over the diplomatic push: What does North Korea want in exchange for a promise to denuclearize, and what is the United States willing to give?

The query underscores the challenge U.S. officials are facing in the two weeks before President Trump, if all goes as planned, will hold an unprecedented meeting with his North Korean counterpart, Kim Jong Un.

Trump has boasted that North Korea already has made significant concessions, including the release of three American prisoners, without getting anything in return from the United States, although analysts said that having a chance to sit across from the U.S. president would reward Kim by elevating his global standing.

But experts say the key to the North’s willingness to scale back its nuclear program will be the administration’s ability to provide the Kim regime a sense of security, in addition to economic and political incentives.

Speaking to reporters last week, Trump vowed to “guarantee” Kim’s safety under a nuclear deal, saying: “His country will be rich, his country will be hard-working and very prosperous.”

Secretary of State Mike Pompeo, summing up on Thursday two days of talks in New York with a senior North Korean envoy for Kim, said progress had been made toward holding a meeting that he said would provide a “once-in-a-lifetime opportunity” to end the nuclear threat.

U.S. negotiators have not announced an agenda for the summit, which could bring the two leaders together in Singapore on June 12.

Former U.S. officials said Kim’s overarching objective is aimed at achieving what his father and grandfather were not able to do: break what the regime has called the United States’ “hostile policy” that has existed since the Korean War armistice in 1953.

That goal has been elusive despite attempts over the past quarter-century to cement lasting deals to freeze or end North Korea’s nuclear program. Each of those initiatives, backed by Presidents Bill Clinton, George W. Bush and Barack Obama, foundered as U.S. officials accused North Korea of cheating and advancing its nuclear and missile programs.

The efforts include a 1993 deal to keep North Korea in the Nuclear Non-Proliferation Treaty; a 2000 agreement to freeze missile development in exchange for relaxed sanctions; the inconclusive “six-party talks” that ended in 2009; and a “Leap Day” deal in 2012 to freeze nuclear and missile testing.

With that checkered history in mind, Kim declared in a New Year’s speech that North Korea had reached nuclear proficiency and said the regime would turn its attention to economic development. Foreign policy analysts interpreted that as a sign that Kim believed he had new leverage to win concessions from global powers.

“The top-line ambition of North Korea is respect through diplomatic normalization, an end to the political pressure and economic pressure tactics to change their nation, and acceptance as they are into the international system,” said Frank Jannuzi, president of the Maureen and Mike Mansfield Foundation, who made three trips to North Korea while working as a Democratic aide on the Senate Foreign Relations Committee.

Experts said a summit might lead to an effort to forge a peace treaty formally ending the war and establish diplomatic relations in the form of a liaison office in the respective capitals.

Kim is also likely to demand that the United States remove North Korea from its list of state sponsors of terrorism, a designation that was reimposed last fall. Such a move would be mostly symbolic, given the breadth of economic sanctions that would remain in place until the North took verifiable steps toward denuclearization.

But former officials expressed frustration that such confidence-building measures have been met with disinterest from the North Koreans in the past. Christopher Hill, who led the U.S. delegation in talks during the George W. Bush administration, said the Bush White House, after initial skepticism, agreed to the concept of opening liaison offices. But when Hill proposed it, the North Korean side rejected the idea.

“The trouble with things the North Koreans say they want, they want them until they get them, then they don’t care,” Hill said.

Experts said coordinated international economic pressure on North Korea, key to Trump’s “maximum pressure” strategy, could already be weakening as Kim reaches out to Beijing and Seoul.

Trump has contrasted a path toward prosperity for the North under a nuclear deal with the potential downfall of the regime if talks collapse. But Jean Lee, a Korea analyst at the Wilson Center in Washington, noted that South Korean President Moon Jae-in has been careful to frame talk of benefits to Pyongyang in terms of an “economic partnership.”

“If we characterize it as a transactional deal where North Korea, as a poor country, stands to gain economically if it gives up its nuclear program, that’s not going to work,” Lee said. “Phrasing it that way, treating it like a business deal . . . won’t give their leader much room to spin it.”

Suggestions from Vice President Pence and national security adviser John Bolton that North Korea should not expect reciprocal economic benefits until it fully forfeits its nuclear program elicited a fierce response from Kim aides, who threatened to cancel the summit last week and warned of a potential nuclear showdown.

Even more important than economic incentives for North Korea will be security guarantees assuring Kim, who is in his mid-30s, that his authoritarian regime can survive for decades.

Although the U.S. force of about 28,000 troops in South Korea has long been a core North Korean complaint, experts said that its negotiators, aware of U.S. and South Korean officials’ insistence that removing U.S. troops is off-limits for now, were unlikely to push immediately for their removal and would focus instead on the peace treaty.

“Once that is concluded, it will undermine the rationale for the troop presence,” said Sue Mi Terry, who tracked Korea at the CIA. “Why do we need to be there if the war has ended?”

An eventual withdrawal might even suit Trump, who has suggested that he might remove U.S. troops from bases in allied nations, including South Korea and Japan, that he says do not shoulder enough of the cost.

The military exercises the United States conducts with South Korea are a more likely short-term target for North Korean officials, who depict them as rehearsals for an invasion or regime change. Pyongyang objects particularly to the participation of advanced and nuclear-capable weaponry in any exercise, such as the F22 stealth fighter and nuclear-capable naval assets.

Adjustments to training and exercises probably would pose no serious threat to the U.S. military posture, analysts said, while rewarding Kim with a political win at home.

“It would be helpful to consider ways to modify those missions,” said Frank Aum, a former Defense Department official.

The Pentagon might, for example, alter training missions it conducts for B-52 bombers located in Guam and B-2 stealth bombers, which occasionally have flown over the Korean Peninsula, and reconsider sending carrier strike groups to waters off the Korean Peninsula.

The risk, however, is that a deal that reduces the American military footprint in the region could leave the United States less prepared for future conflicts in Asia while increasing South Korea’s vulnerability to conventional attacks from North Korea, said Abigail Grace, who worked on Asia policy at the White House until earlier this spring.

“The biggest danger is that the U.S. will be tempted to trade conventional force posture for nukes,” said Grace, who is now at the Center for a New American Security.

But all of these potential trade-offs probably would come down the road.

Daniel Russel, a top Asia policy aide in the Obama administration, said the rushed summit preparations meant that the two sides probably would try to reach broad, mostly symbolic agreements on denuclearization and peace and leave it to aides to work out the details.

“They will announce they have looked into each other’s eyes and found each other trustworthy,” said Russel, now at the Asia Society. “And they’ll announce they have decided to direct their deputies to begin a negotiating process. But when you scrape the gold paint off, lo and behold, there’s not much there.”