There are plenty of lessons we can learn from the 9/11 Commission report. But perhaps the most important is that serious threats don't appear overnight -- they grow over time. And our ability to respond must change and grow as well.

For example, al Qaeda started planning for 9/11 in late 1998 and early 1999. In theory, the attacks could have been prevented at many points if we had taken aggressive action. But we never did.

Today, another long-term threat is growing: North Korea is doing everything it can to develop intercontinental ballistic missiles. South Korean media reports indicate that North Korea is testing a land-based missile that could travel up to 3,700 miles. That would put parts of Alaska at risk. Unfortunately, there's more.

The authoritative journal Jane's Defense Weekly recently reported that the North Koreans also have acquired a dozen decommissioned Soviet submarines -- complete with sea-based missile systems. Those weapons "would fundamentally alter the missile threat posed by the DPRK (Democratic People's Republic of Korea) and could finally provide its leadership with something that it has long sought to obtain -- the ability to directly threaten the continental U.S.," the editors wrote.

Luckily, while this threat has been building, we haven't been standing still.

On the diplomatic front, the U.S. has joined China, Japan, South Korea, Russia and North Korea for several rounds of negotiations. These "six-party talks" are aimed at finding a peaceful way to convince Pyongyang to give up its nuclear ambitions.

But more importantly, we're also taking military action. After decades of argument and political infighting, we're finally building a missile defense system.

Last month, the first ground-based missile interceptor was installed at Fort Greely, Alaska. As many as five more will be in place by year's end. But that's not the end of the process. It is, as Winston Churchill might have said, merely the end of the beginning of the process.

The ground-based interceptors aim to blow up ballistic missiles during the middle of their flight, before they start plunging toward earth (their "terminal phase"). Soon the ground-based weapons will be augmented by a number of sea-based rockets on navy ships. That system is scheduled to be in place by the end of 2005. These are both parts of a "layered" defense capability that will eventually allow us to take several shots at destroying an incoming ballistic missile.

But we can't stop here. Congress also should press ahead with research into what's called "boost-phase" programs. These programs aim to shoot down an enemy missile at the beginning of its flight, rather than waiting until it's on the way back down. The boost phase is generally the best time to attack a missile, because it's moving fairly slowly and generating plenty of heat as it blasts off.

Unfortunately, boost-phase research is likely to come under fire because of a Congressional Budget Office report that an early interceptor program may cost billions of dollars, in fact between $16 billion and $78 billion.

But Heritage Foundation expert Baker Spring says that estimate includes the entire 20-year cost to procure and operate boost-phase systems. Boost-phase research is still in early development, far behind later-stage missile defense, and preliminary testing will cost far less. For example, five years ago The Heritage Foundation's Commission on Missile Defense presented a plan to begin fielding space-based interceptors for about $5 billion.

The final price tag is impossible to predict yet. But because of the importance and promise of boost-phase defenses, we should press ahead with the tests and see how effective the program is. That way Congress will be able to determine if the technology merits greater investment over the long term.

The Bush administration is building a layered missile-defense system that will help protect us from the threats of the 21st century. Those threats will grow stronger over time. But as long as we keep developing our missile defenses, we'll be able to respond effectively.

Ed Feulner is president of The Heritage Foundation (heritage.org), a Washington-based public policy research institute.