Could North Korea ever give up the bomb?
To state the obvious: North Korea is not Iran.
There are crucial differences which make defusing Kim Jong-un's nuclear ambitions much harder than removing those of the Iranian leadership.
Firstly, North Korea already has the bomb - and dismantling a machine which exists is a lot harder than preventing its creation in the first place.
The genie is out of the bottle.
North Korea has already conducted three tests of devices (detected by seismologists). It boasts of its "nuclear deterrent" in state-run media.
Accordingly, the important question is not about if but when - in particular, when might it be able to make a bomb small enough to put on top of a missile capable of leaving the earth's atmosphere and re-entering to hit distant targets.
South Korea and Japan already feel the threat acutely because North Korea has demonstrated the use of shorter-ranger missiles, and the US fears that a missile capable of reaching the West Coast isn't far away.
The second crucial difference between the Iranian and North Korean situations is that Iranians have some power over their elected leaders through the ballot box, even if they cannot remove the Supreme Leader Ayatollah Ali Khamenei, who recently voiced his concern over any deal on his country's nuclear programme.
However you view the state of democracy in Iran, there is pressure from below to deliver satisfactory economic conditions.
If the people feel government policy is failing them, they can put pressure on that government to a degree unimaginable in despotic North Korea.
This means that sanctions are much more likely to work against Iran than North Korea where the people have to suck up whatever policy Kim Jong-un dishes out.
It is impossible to know from outside exactly where North Korea stands in its pursuit of nuclear weapons. There may be sabre-rattling.
Some experts believe, for example, that fearsome missiles shown off in parades may be fakes for Western eyes.
Intelligence services and politicians in South Korea and the US may have an interest in talking up the threat.
Sympathisers with North Korea may want to talk it down.
So what's our best knowledge?
Here's how Siegfried Hecker of Stanford University, one of the world's acknowledged authorities, put it: "North Korea's nuclear weapons programme has been relentlessly expanding for a decade, and poses a real and deadly threat to the rest of Northeast Asia".
Prof Hecker, a former director of the Los Alamos National Laboratory in the United States, visited North Korean nuclear plants before outsiders were completely barred.
He wrote recently in the Bulletin of the Atomic Scientists: "During my first visit to North Korea in January 2004, North Korean officials were eager to show my Stanford University colleagues and me the plutonium bomb fuel they produced following a diplomatic breakdown with the George W Bush administration.
"Four years ago, during my seventh visit to the country and two years into the Obama administration, they surprised us with a tour through an ultra-modern centrifuge facility, demonstrating that they were capable of producing highly enriched uranium, the alternate route to the bomb".
World's nuclear powers:
US: First nuclear test July 1945
Russia: August 1949
UK: October 1952
France: February 1960
China: October 1964
India: May 1974
Israel: Suspected September 1979
Pakistan: May 1998
North Korea: October 2006
The ability to produce this highly enriched uranium changed the calculation because it meant that North Korea wasn't constrained by the supply of plutonium.
It opened a second route.
Prof Hecker said: "The plutonium produced in the early 1990s had been tied up for almost a decade in spent fuel, which was stored safely with US assistance and kept under international inspection. Today, North Korea may possess a nuclear arsenal of roughly 12 nuclear weapons, half likely fuelled by plutonium and half by highly enriched uranium."
If that's the state of the bomb-making capability, what about the means to deliver it?
North Korea certainly has missiles capable of hitting South Korea, Japan and territories in the Western Pacific but has not demonstrated the ability to send a rocket into space and then to re-enter the earth's atmosphere and hit the continental US (an inter-continental ballistic missile).
Here's how the US Department of Defense assessed the situation two years ago (though clearly North Korean research will have moved on): "North Korea followed its 12 February 2013 nuclear test with a campaign of media releases and authoritative public announcements reaffirming its need to counter perceived US 'hostility' with nuclear-armed ICBMs (inter-continental ballistic missiles).
"North Korea will move closer to this goal, as well as increase the threat it poses to US forces and Allies in the region, if it continues testing and devoting scarce regime resources to these programmes.
"The pace of its progress will depend, in part, on how many resources it can dedicate to these efforts and how often it conducts tests".
Which brings us to the politics.
There are signs that Russia and China wants to get back to the six-party talks (involving them, the US, Japan and the two Koreas) which broke down in 2009 when North Korea pulled out.
And according to China's Xinhua News Agency, North Korea has also indicated it wants to negotiate.
The US does not believe any negotiations would be in good faith because it sees no evidence of what its diplomats call "meaningful steps" to demonstrate a real wish to renounce nuclear weapons.
The rogue card in all this is how much Kim Jong-un needs the outside world if he wants to raise the living standards of his citizens.
Three years into his rule, few can see signs of any softening of the belief that possession of the bomb gives him immense clout.