A North Korean nuclear- weapons test, taken in isolation, is bad enough. But put into a wider context, the underground blast over the Memorial Day weekend is worse than many realize.

A lot worse.

First, on the political front, North Korea's Kim Jong Il has challenged President Obama more in four months than he did President George W. Bush in eight years.

Since Obama has taken office, North Korea has kicked out UN nuclear inspectors, launched both short- and long-range missiles and tested a nuclear weapon.

It's not clear why the dictator has chosen to badger Obama, especially considering the president's promises of a kinder, gentler touch when it comes to rogues. But it's definitely not good news for Uncle Sam -- and the conclusion has to be that more provocations are coming in our direction.

The question is: When and how big will the next one be?

Second, this nuclear test appears to have been more successful than North Korea's first in October 2006.

The analysis of the test continues, but we already know that this plutonium-based weapon's yield (that is, its explosive power) was greater than that of the first, running perhaps in the 2- to 8-kiloton range. (A kiloton equals the explosive force of 1,000 tons of TNT.)

The situation appears even darker upon consideration that the blast comes in the shadow of North Korea's launch just last month of a Taepodong -- a missile thought to have intercontinental-range potential.

The missile shot, conducted under cover of a satellite launch, wasn't a complete success. But it did demonstrate some key ICBM building blocks, such as using multiple stages for long-range flight.

Adding to that concern, US intelligence believes that "North Korea may be able to successfully mate a nuclear warhead to a ballistic missile."

In other words, the threat that a North Korean nuke could reach us is on the horizon. We don't know how distant that horizon is, but it appears to have moved closer last weekend.

Third, North Korea's successes make it a popular merchant for those that want to obtain nukes and missiles, raising concerns that technology, material and know-how will proliferate among anti-American actors.

Pyongyang was caught red-handed once by the Israelis, who in September 2007 destroyed a nuclear facility the North Koreans were building for the Syrians.

North Korea, which has been trading ballistic missiles with Tehran for some time now, could help move along the Islamic Republic's atomic ambitions, too.

We certainly don't want that pair training ICBMs on us.

Sure, some Western analysts are saying this blast is but another of the irascible Kim's "I will not be ignored" moments, but it's really a lot more serious than that.

In light of the string of setbacks experienced since Obama took office, his policy of engaging with North Korea (and other rogue regimes) has to be considered under severe strain, if not failing.

Sure, you can try more diplomacy, more UN resolutions and even more economic sanctions to get North Koreans back to the negotiating table. But time isn't on our side.

We must be able to defend ourselves from a position of strength against the prospect of more North Korean belligerence, including a nuclear-capable ICBM arsenal.

Unfortunately, changes in our defense posture and budget, such as declining efforts on US missile defense, appear to be putting that goal in jeopardy.

With the North Korean missile and nuclear threat growing, the need for a more robust missile defense has never been greater.

It's time the Obama administration realizes this -- and makes the continued development of missile defense integral to any strategy for dealing with the North Korean challenge.

Peter Brookes is senior fellow for National Security Affairs in the Davis Institute at The Heritage Foundation.