The vast majority of Americans believe that the United States can defend itself against a ballistic-missile attack.

They're wrong. But, thankfully, that's changing.

In as little as 45 days, we will finally have a limited capability to shoot down incoming intercontinental ballistic missiles.

With Iran and North Korea threatening a nuclear (weapons) breakout and China modernizing its entire array of nuclear arms, the Bush administration's historic achievement of fielding an operational missile defense comes not a moment too soon. Like terrorism, the ballistic-missile threat is real. Roughly two-dozen countries already possess ballistic missiles. Many are pursuing weapons of mass destruction (WMD) as well. A ballistic missile mated with a WMD warhead could be used to intimidate, coerce or blackmail the U.S., its friends or allies.

Most pressing, North Korea already may have two to three operational nuclear weapons. And North Korea's "Dear Leader," Kim Jong Il, is working feverishly to combine them with missiles capable of reaching the continental United States.

No less troubling, American intelligence believes that Iran will be a nuclear-weapons state by the end of the decade. (Israeli says by 2007.) With North Korean help, the mullahs of mayhem are working on an intercontinental ballistic missile able to reach the American mainland.

And Asia's 900-lb. gorilla, China, is rapidly increasing the size of its ballistic missile arsenal. Beijing also is modernizing its long-range nuclear forces, making them more lethal, survivable and accurate.

None of this is good news. But in a largely unheralded stump speech last Tuesday at a Pennsylvania Boeing plant, President Bush reaffirmed a campaign promise he made four years ago: to deploy a national missile defense.

He told the cheering crowd of defense workers that America's first ballistic-missile interceptor had been loaded into its underground silo at Alaska's Fort Greely last month. Four more interceptors will be on duty by the end of the year, giving the U.S. a limited missile defense against the likes of North Korea.

An additional 10 interceptors will be active by the end of 2005, including four at Vandenberg Air Force Base, California. The initial deployment will also enable the Pentagon to continue technical development for handling more robust missile threats like China.

Bush's plan wisely calls for a layered approach for defending against all ballistic-missile classes.

For instance, sea-based interceptors will be deployed on Navy Aegis-class ships to defend against both short- and medium-range ballistic missiles. These ships will not only help protect the U.S., but can also provide missile defense to friends and allies around the world. For the shorter-range missile threat, the Patriot missile system (of Persian Gulf War I fame) will protect our troops and allies in the field overseas.

Missile defense hasn't been without strong critics. Some have said that the technology was impossible - that the equivalent of hitting a bullet with a bullet in space was a pipe dream. Successful testing has proven them wrong. And the technology over time will only improve.

Others have said missile defense is prohibitively expensive. (John Kerry is calling for reducing missile-defense spending.) But after the horrors of 9/11, is $50 billion over five years too expensive to prevent a similar or greater catastrophe? Hardly.

There is no sense in holding ourselves deliberately hostage to nuclear terror if there are alternatives. When whiz-bang anti-missile technology is available, clinging to the Cold War paradigm of mutually assured destruction (MAD) is just that - mad.

Deploying missile defenses gives us a broader range of policy options beyond military strikes or massive nuclear retaliation. And further developing missile defense might actually dissuade some from pursuing WMD and missiles at all.

The proliferation of ballistic missiles and WMD is a growing challenge to American - and international - security. Missile defense is just one aspect of a comprehensive strategy for dealing with the threat. Nonproliferation treaties, interdiction operations and multilateral export controls also come into play. By the end of this year, we will have begun to fulfill President Ronald Reagan's 1983 vision of ending the morally - and strategically - bankrupt policy of keeping the American people deliberately vulnerable to ballistic-missile attack.

Ongoing missile defense efforts will make us more secure in an age of seemingly endless insecurity. We can only hope that a strong missile defense will have the same effect on repressive regimes as Reagan's stoic vision for a missile defense had on the Soviet Union.

Peter Brookes, a Heritage Foundation senior fellow, served in Latin America while on active duty in the U.S. Navy.