Thanks to a recent declaration by the head of the Pentagon's Missile Defense Agency on how to proceed with construction of a missile shield, an effective sea-based defense could be up and running as early as 2004. Previously, officials were estimating that no system would be in place before 2008, if then.

But now the agency, led by Air Force Lt. Gen. Ronald T. Kadish and made up of military and civilian personnel, has acknowledged that a "sea-based midcourse" program-interceptors mounted on ships that would target missiles in the middle of their flights-could be put on a fast track.

Outside experts concluded that sea-based options offer not only the quickest route to a missile defense but the least expensive. The proposed platform for a sea-based system -- the Navy's Aegis cruisers -- already exists. And the Aegis system, which has protected the U.S. fleet since the 1970s from cruise missiles and aircraft, has shown that it works.

Another advantage: Ships bearing sea-based interceptors can be positioned near sites from which potential attacks could emanate. The interceptors could hit approaching missiles as they reach "cruising altitude" -- before decoys and warheads could be released. And if the first wave of interceptors doesn't take down incoming missiles, others later in the planned "layered" defense (which would include land- and space-based elements) still could save the day.

This isn't a conservative or Republican position. It's shared by virtually everyone --military and civilian -- who has studied the problem closely. Defense Department researchers concluded in 1999 (in a report that echoed a recommendation made by The Heritage Foundation in 1995) that a sea-based system using Aegis cruisers would be capable of seeking and destroying long-range missiles.

Admiral Jay Johnson, then the Navy's top officer, said as much two years ago. And former Defense Secretary Harold Brown and former deputy defense secretaries John Deutch and John White, all of whom served in Democratic administrations, also have said that a sea-based system would be "cheaper and technically less risky" than a ground-based system. Sea-based interceptors have passed two tests this year, and more than a few missile-defense skeptics now concede that America has the technological know-how to pull this off.

The demise of the Anti-Ballistic Missile (ABM) Treaty -- America's withdrawal became official June 13 -- also should speed up the process. For one thing, it's now permissible to deploy a missile-defense system (the treaty prohibited a national defense), which should provide a significant jump-start by itself. And no ABM Treaty means no restrictions on how or what we test.

The events of Sept. 11 brought new urgency to the task of finding a way to protect the American homeland and U.S. allies from ballistic attack. Would a missile-defense system have thwarted the means of attack employed that day? No. But the highly organized terror groups that threaten us -- not to mention states such as Iraq, Iran and North Korea that are trying to develop weapons of mass destruction and the means to deliver them anywhere in the world -- probably will have other schemes in mind when next we encounter them.

It's highly unlikely, given the open talk in Washington of putting Iraq under new management, that Saddam Hussein will wait long after he acquires the means to deliver a weapon of mass destruction -- such as a chemical, biological or nuclear-tipped missile -- to do so. That's why, although a layered system of land-, sea- and space-based interceptors should remain America's goal, creating a mobile, versatile first front against missile attack must take top priority.

America finally has found a reliable path to missile defense and the enormous level of security it could supply. The challenge now is to maintain the momentum. Time, unfortunately, isn't on our side.

Baker Spring is the Kirby research fellow in national security policy at The Heritage Foundation, a Washington-based public policy research institute.