People watch a TV broadcasting a news report on the upcoming summit between the U.S. and North Korea, in Seoul, South Korea, June 11, 2018. Kim Hong-Ji | Reuters

President Donald Trump has lowered expectations for Tuesday's summit since declaring in April that North Korea "agreed to denuclearization." Now he says there may not be any agreement. "The minimum would be relationship," Trump says of his sit-down with Kim Jong Un. "We'll have met each other. We will have seen each other. Hopefully we will have liked each other." That recalibration reduces the risk of disaster in Singapore. Dashed hopes for much more – immediate nuclear disarmament by North Korea — could ramp up tensions rather than reduce them. So what are realistic expectations? The latest NBC News/Wall Street Journal poll shows American voters have no clear view: 26 percent say Trump will ask too much; 10 percent say he'll give up too much; 14 percent expect him to get a deal tilted toward the U.S.; 17 percent expect a deal fair to both sides. The largest group, 33 percent has no opinion. I asked foreign policy veterans in both parties. Their views aren't all that different, reflecting the frustrations Democratic and Republican presidents have shared over North Korea in recent decades. Their answers converged around a few possibilities.

Best-case outcome

"Happy talk," as Jim Steinberg, a former advisor to presidents Bill Clinton and Barack Obama put it — and a little more. If the two leaders establish rapport and confidence, they would direct their aides to begin gritty negotiations on the details that matter. Those are how and when North Korea would reduce and/or eliminate its nuclear weapons, and how and when the U.S. would enhance Pyongyang's security and prosperity. One acronym signaling success would be CVID — "comprehensive, verifiable, irreversible denuclearization." Getting North Korea to embrace it for the first time would represent a breakthrough, even if Kim insists that it apply throughout the peninsula including South Korea. Peter Feaver, who advised in President George W. Bush, says the best-case outcome requires a concrete "meaningful concession" from Kim. That would demonstrate the promise of future talks. One possibility: in addition to freezing nuclear tests, a North Korean agreement to dismantle one or two of its existing nuclear bombs. Another: commitment to a specific denuclearization time-frame.

Worst-case outcome

Longtime diplomats fear two scenarios. One is that the mercurial American president, who just erupted in fury at Canadian Prime Minister Justin Trudeau, perceives failure or disrespect from Kim that draws him toward military conflict. Trump calls this moment for diplomacy "a one-time shot." The North Korean regime has declared outright it won't accept unilateral denuclearization. Trump may privately expect he'll achieve that, anyway. "A worst-case outcome is clearly a return to a path toward possible war, like back in 2017, because of irreconcilable differences about a deal combined with a breakdown in how they get along," says Michael O'Hanlon, who advised President Clinton. The other worrisome prospect is what Richard Haass, who advised both presidents Bush, calls "catastrophic success." In this scenario, Trump's hunger for an historic triumph leads him to give too much, such as withdrawal of U.S. troops that protect South Korea and project American power in the region. "A deal to leave the Korean peninsula without any verifiable nuclear rollback" represents the worst-case, says Danielle Pletka, a former Republican Senate aide at the American Enterprise Institute. Last weekend's blow-up at the G-7 in Canada, by making Trump more eager for success than he was already, leaves veteran diplomats worrying that odds of this result have risen.

Most likely outcome