The old definition of insanity is doing the same thing again and again and expecting a different result. The new definition, which applies only in the case of North Korea, is: doing something different and expecting a different result.

The North Koreans have been pursuing a nuclear arsenal for a couple of decades. We have tried negotiating; we've tried cutting off negotiations; we've tried threatening them; we've tried ignoring them, and we've tried whistling "Camptown Races" while standing on our heads. Nothing has made any visible difference. Come rain or come shine, they keep pursuing a nuclear arsenal.

So it should have come as no surprise when the Pyongyang government conducted a nuclear test, the third it has done. Given the regime's record, it was not a matter of if it would light the fuse but only a matter of when.

Maybe it was significant that the explosion took place the same day as President Barack Obama's State of the Union address. Maybe it was significant that it occurred before rather than after the inauguration of a new South Korean president. Maybe it was an auspicious day in the eyes of the regime's astrologers and Tarot card readers. Or maybe none of this stuff played any role whatsoever.

As for why, that question is easier. The common assumption is that North Korea is playing a high-stakes game aimed at impressing or coercing the United States, or South Korea, or Japan, or China or its own populace. But the better explanation is the simpler one. It does these things because it wants a nuclear weapon more than anything else in the world.

Why? Same reason we wanted one. Same reason the Soviet Union felt impelled to follow suit. Same reason China did likewise.

Same reason all the other existing nuclear powers insisted on getting nuclear weapons: because they have a value that is both enormous and unparalleled. They tell other countries, "You cannot eliminate us except by assuring your own destruction."

North Korea has obvious reasons to prize this guarantee. To begin with, it is small, poor, backward, short on friends and long on enemies. Those facts have always furnished excellent arguments for the security blanket offered only by the bomb.

But in this century, the incentive got sharper. In 2002, following the 9/11 attack, President George W. Bush described North Korea as part of an "an axis of evil arming to threaten the peace of the world." The other members were Iraq and Iran. The U.S. proceeded to invade Iraq to topple its government, and it called for "regime change" in Iran, as well—with some people in Washington favoring military action to bring it about.

The North Koreans had no trouble counting to three, and they didn't need a map to see who might come next. Defense Secretary Donald Rumsfeld confirmed their fears with a secret 2003 memo, leaked to the press, endorsing regime change there, as well.

President Bill Clinton's administration coaxed the North Koreans into signing an agreement to halt their nuclear program. But they cheated on the deal, and when the Bush administration exposed the fraud, the U.S. initiated a tougher policy. That didn't work either. In 2006, Pyongyang carried out its first nuclear detonation.

When Obama took office, his secretary of state made conciliatory gestures toward North Korea. What happened? Oh, you can guess. Within months, it launched a long-range missile and carried out a second nuclear test.

The assumption among both doves and hawks is that there is some action we can take that will show the regime the error of its ways. Hawks are the latest to have their turn: An editorial in The Wall Street Journal urged the president to threaten military strikes so Pyongyang knows "it faces a choice of giving up the bomb or failing."

But that's not a credible threat. The regime has outlasted many forecasts of failure. And it can respond to any attack by using one of its nuclear weapons. But it doesn't even need that option: With a mass of artillery and rocket launchers within range of Seoul, it is fully capable of turning the capital into a "sea of fire," to use its charming phrase.

The North Koreans are staunchly resolved to build a nuclear arsenal. We may entertain fantasies that we can stop them. But they know better.