The once unthinkable is imminent. On June 12 at 09:00 local time, at the Capella Hotel on Singapore's resort island Sentosa, the top leaders of the United States and the Democratic People's Republic of Korea (DPRK, North Korea) will meet for the first time in history.

What will Donald Trump and Kim Jong-un talk about? Many things, no doubt. But first and foremost, denuclearization.

Meaning? Aye, there's the rub. The "D word" has very different connotations for the two sides. The gap between them had looked not only unbridgeable but, so to say, unfudgeable.

The trumpet Trump had been blowing, and still his avowed stance, was a hand-me-down from George W Bush: CVID, meaning complete, verifiable, irreversible [nuclear] disarmament, by North Korea. Actually a more accurate acronym would be CVIDN, N standing for Now.

Trump's team sneered at the old game of interminably slow salami-slicing, as in the Bush-era Six Party Talks (6PT) which began in 2003 and fizzled out in 2008. Their model was Libya – meaning a single neat one-time event, as when Muammar Qaddafi surrendered his entire fledgling WMD programme to the US and UK early in 2004.

John Bolton, Trump's hawkish national security advisor who especially stressed Libya, is no fool. He knew full well that to North Korea Libya meant what followed: Qaddafi's overthrow and grisly murder by NATO-backed insurgents. (Some reckon Bolton deliberately sought to sabotage the Kim-Trump summit; latest reports suggest he has been sidelined on this issue.)

The Libya model is not merely tactless; it is technically unworkable. Qaddafi only had a few bits and pieces, 25 tons in all, which could all easily be flown out to Oak Ridge, Tennessee.

North Korea, by contrast, has a far more advanced programme with "dozens of sites [and] hundreds of buildings". Thus former Los Alamos director Siegfried Hecker, who has seen more of the DPRK's facilities than any other American. In a new report offering a detailed road map for denuclearizing North Korea, Hecker estimates this could take 15 years.

So US impatience is impractical. Impolitic too, given that North Korea has always posited denuclearization as a phased process, not a one-off event. On May 31 Kim Jong-un reiterated this long-held stance, telling Russia's visiting foreign minister Sergei Lavrov that he hopes "DPRK-US relations and the denuclearization of the Korean peninsula will be solved on a stage-by-stage basis." (Note the explicit linkage).

Had both sides stuck to their guns, there would be an impasse. Guess who moved. On June 1, meeting Kim's special envoy Kim Yong-chol at the White House, Trump changed his tune. Now the summit is just a beginning. "We’re not going to go in and sign something on June 12 and we never were … We're going to start a process … I told them today ‘Take your time'".

There go Trump's boasts of being different from his predecessors. Bill Clinton also started a process with North Korea, as did – eventually – George W Bush. (Obama, not so much.)

This solves one problem, but another remains. Who must denuclearize? For the US and the world, as per numerous UN Security Council resolutions (all unanimous), it is North Korea whose nuclear and ballistic missile tests are in flagrant breach of international law.

Pyongyang, needless to add, sees it differently. If they yield, so must their enemies. Less clear is what exactly the DPRK demands as a quid pro quo. Previously, as 38North's Joel Wit has reminded us, they sought a formal treaty ending the 1950-53 Korean War – with the corollary that all the 28,500 US troops based in South Korea must be withdrawn.

That is a non-starter – or rather a bilateral matter for the ROK and US. It is good to hear from Seoul, which under Moon Jae-in sees itself as an intermediary, that North Korea has dropped that demand. Better yet would be for Trump to hear it from Kim Jong-un. A peace treaty on its own is hardly problematic; I expect the Singapore summit will set that prospect in train.

But what of the nuts and bolts of denuclearization? The Punggye-ri stunt fooled no one. North Korea allowed no expert inspections, and can dig test tunnels elsewhere any time it chooses.

After a long history including the 1994 US-DPRK Agreed Framework and the 6PT, both of which included an intrusive role for inspections by the International Atomic Energy Agency (IAEA), the DPRK well knows what a serious denuclearization effort would involve. Less clear, as always, is how much it is prepared to concede, how soon, and on what terms.

With both sides now agreeing this will be a drawn-out process, we are back to the same old game. Holding a summit up front is certainly novel, but this may be a damp squib unless something solid comes out of it.

Might Kim Jong-un yield up a few bombs? That would help, but don't bet on it. The fact that the Panmunjom talks on the summit agenda are now (June 6) on their sixth day bespeaks how hard it still is to align the two sides'Venn diagrams to get even a sliver of overlap.

Trump's shtick is to be different. Last year all too ready to contemplate war, he is now eager to clinch a deal – maybe any deal, at any price. Last week he briefly cancelled the summit, but that was a ploy. And it worked. Both sides now seem committed to this meeting, which means they have to come up with some meat which they can each present as substantive and a win. Like everyone, I am curious to see what this rabbit from the conjurer's hat will look like.

Kim Jong-un has surprised us before, and may do so again. Yet I expect that after Singapore, like before, denuclearization will remain an agreed goal – but a distant one. If anyone still gets classical allusions, sceptics who reckon North Korea's game is basically to prevaricate "until the Greek calends" (alle calende greche) have yet to be proven wrong. How good it would be if they were.