It would be child's play to take out North Korea's nuclear facilities in a single coordinated strike. The North Korean air force is not modern enough to stop U.S. or Russian or even Chinese strike aircraft. The country's few nuclear weapons are not deliverable by missile yet, so even if one or more of them did survive the first strike, Pyongyang could not hit back with nukes.

So why don't the countries that worry about North Korea nuclear weapons skip the endless haggling with a regime that does not bargain in good faith, and just use their superior weapons to strike the nuclear card from North Korea's hand? Surely they aren't afraid of a conventional land invasion of the South by the North.

The North Korean army is large, but without air cover it would be torn to shreds in a matter of days, or even hours. This is the 21st century, and an army that cannot protect itself from air attack is just a bunch of dead men walking. There must be some further consideration that keeps the option of a preventive attack on North Korea off the table.

There is. It is called China.

It has been a very long time since Mao Tse-tung declared that China and North Korea were "as close as lips and teeth". Today's Beijing has little sympathy for a fellow Communist regime in Pyongyang that is not only brutally repressive but also an abject economic failure. North Korea has even reverted to dynastic rule, and other medieval phenomena like famine have become chronic there.

North Korea is an embarrassment to the Communist system that the Chinese regime uses to justify its own monopoly of power. Nevertheless, the Beijing regime cannot run the risk of letting Kim Jong-il's moth-eaten regime simply collapse, which would be the probable result of a successful disarming strike against Pyongyang's nuclear weapons.

Regime collapse in Pyongyang would send a flood of destitute North Korean refugees across the frontier into China, and they might carry the infection with them. What China worries about is regime collapse in Beijing.

It is 20 years this week since the pro-democracy movement in China was crushed when troops and tanks swept onto Tiananmen Square on June 4, 1989 and massacred hundreds or even thousands of students.

The regime officially dismisses the protesters who camped peacefully on the square for weeks as "hooligans", but it is still haunted by the fear that the Chinese people might some day demand their country back.

On the surface, it seems unlikely that they will demand it soon, for the Communist Party's strategy of buying the population's loyalty with high-speed economic growth has been a runaway success. Even during the worst global recession in half a century, China is officially forecasting 10 percent economic growth in 2009.

But what if the goose stops laying the golden eggs? It's one thing to be facing economic hard times in Berlin or Washington or Cape Town, where the government's legitimacy comes from democratic consent. It's another thing to be a government facing economic hard times when your only legitimacy comes from economic good times.

Even in good times, the Chinese government is acutely aware that it is among the last surviving Communist regimes in the world, and that the ideology on which it bases its right to rule is essentially dead in the eyes of the people it rules. It could face a potentially fatal challenge very fast if things went wrong, and it knows it. That was what happened in 1989.

Right-thinking liberals insist that the regime overreacted in 1989; if it had agreed to talk to the students instead of killing them, everything would have been all right. Zhao Ziyang, then general secretary of the Communist Party, who was dismissed and put under house arrest for the rest of his life, believed that to the day he died: "Most people were only asking us to correct our flaws, not attempting to overthrow our political system".

Maybe that is what most people wanted in Tiananmen Square in June 1989, but if the regime had started to make concessions it would have been gone by the end of the year. That was what happened in Poland, East Germany, Czechoslovakia, Romania, and eventually even in the Soviet Union itself. It would have happened in China, too.

The lesson that the Chinese Communist Party has learned from 1989 is that there must be no more examples of collapsing Communist regimes, especially on China's borders. The danger of infection, however remote, is too great to be tolerated, so North Korea's regime must survive.

Beijing has said that it is "resolutely opposed" to North Korea's nuclear test, but it will not allow the Pyongyang regime to be overthrown. So no disarming strike against North Korea is possible, and the next stage in the crisis is likely to happen at sea when some North Korean ship suspected of carrying nuclear contraband is stopped.

Or you could just have a nasty incident between the fishing fleets jostling for the best positions near the disputed sea border between North and South Korea.

Gwynne Dyer's latest book, Climate Wars, was published recently in Canada by Random House.