Early this month, North Korea claimed to have launched a ballistic missile from a submerged submarine. Last week, it announced it can make nuclear warheads small enough to fit on a missile. If both those claims are true, then it can deliver a nuclear weapon on the United States — at least in theory — but there is always some doubt about North Korean claims.

While a defense official in Pyongyang said Wednesday the country’s nuclear program has “long been in the full-fledged stage of miniaturisation,” some Western defense experts think the North Koreans have not really mastered the art yet. But Gen. Curtis M. Scaparrotti, the senior U.S. military commander in South Korea, thinks otherwise.

“I believe [the North Koreans] have the capability to have miniaturized the device at this point, and they have the technology to potentially actually deliver what they say they have,” Scaparrotti said last October. But to be sure the miniaturised weapon actually works on a ballistic missile, North Korea would have to test-fire it to see whether it survives the heat and vibration of re-entering the atmosphere in working order. It has not yet done that.

Others think the footage of the submarine launch may have been faked. The missile emerges from the sea, sure enough, with the Maximum Leader proudly looking on. But Kim Jong Un obviously was photoshopped in, and in one shot there seems to be a barge floating on the surface near the missile’s exit point. However, let us assume for a moment both claims are true — because they will be, sooner or later.

What does North Korea intend to do with its nuclear weapons? And why is it trying so urgently to persuade its enemies that they are ready to use now?

The rational and conventional answer to the first question is that Pyongyang’s nukes are solely intended to deter the United States from using nuclear weapons on North Korea. The United States has long-standing military alliances with South Korea and Japan, and it has never said it would abstain from using nuclear weapons if there were a war between North Korea and its neighbors.

In this rational world, having enough nuclear weapons to deter the United States from going nuclear would give North Korea a major advantage in the event of a ground war in the Korean peninsula. Its army is much bigger than the South Korean and U.S. ground forces facing it, and it might even manage to overrun South Korea in a non-nuclear war — or, at least, it may believe it could.

How many North Korean nuclear weapons would be enough to deter the United States from using its own nukes, in this context? A dozen would probably do it, and professor Siegfried Hecker of Stanford University, a former director of the Los Alamos National Laboratory, thinks North Korea probably has that many, “half likely fuelled by plutonium and half by highly enriched uranium.”

But rationality has not been the outstanding feature of politics in North Korea recently. In the past three years, Supreme Leader Kim Jong Un has purged most of the men who worked closely with his father, Kim Jong Il, and many have been executed. Whole families have been murdered, including some with links by blood or marriage to Kim’s own.

We cannot just assume North Korea’s nukes are purely defensive or that Kim Jong Un, after 28 years of living in a gilded cage and 3½ years of absolute power, has been adequately instructed in the theories of nuclear deterrence that have become orthodox in older nuclear-weapons states. Nor is anybody in the North Korean military hierarchy going to try to instruct him now, if he is ignorant in such matters.

The simple truth is that the rest of the world doesn’t know what is happening in North Korea at the moment. The mystery has deepened with the abrupt last-minute cancellation of United Nations Secretary-General Ban Ki-moon’s scheduled visit to North Korea. We’ll have to wait to find out what’s really going on — but meantime military forces all over north-eastern Asia undoubtedly are on high alert.

Gwynne Dyer is a London-based independent journalist whose commentary is published in 45 countries.