For almost 30 years, the federal government has left the American people vulnerable to ballistic missile attack.

Most of our leaders over that time considered this a responsible course. We had enough missiles aimed at our rival, the Soviet Union, to destroy it, and the Soviets had enough missiles aimed at us to destroy us. Therefore, the theory went, striking first would only result in one's own annihilation. This was called mutually assured destruction, and it was codified in the 1972 Antiballistic Missile Treaty between the two countries.

Now the Soviet Union is no more, having given way to the weight of history. The treaty joined it on the ash heap of history when President Bush withdrew the United States from its obligations in December 2001.

And within weeks, the country's vulnerability to missile attack will be reduced when Bush declares operational the first elements of a ballistic missile defense for the U.S. This great victory for the American people - and make no mistake, it is a historic achievement and means the federal government will have begun to meet its obligation to defend us to the best of its ability - comes not a moment too soon.

The weapons arrayed against us now may not be as potentially destructive or as numerous as during the Cold War, but the threat of a highly destructive missile attack on the U.S. mainland is almost certainly higher. The report of a 1998 commission chaired by now-Defense Secretary Donald H. Rumsfeld detailed how a larger number of nations, governed by rulers far more unpredictable than any who occupied the Kremlin during the Cold War, have sought ballistic missiles and the nuclear, chemical and biological warheads to arm them.

The devices that will become operational in coming days won't come close to making the U.S. completely secure from missile attack. In fact, they should be viewed more as a testing platform for developing and improving the limited missile-defense capabilities already on line.

Our current systems, all of which are non-land based, can't keep pace with the increasing missile threat. Congress must fund ongoing efforts to strengthen them significantly over the long term. These efforts should focus on space-based interceptors that find and destroy ballistic missiles above the atmosphere en route to their target.

Primarily, this means rebuilding the "Brilliant Pebbles" program pursued by the first Bush administration as part of the Strategic Defense Initiative but canceled by the Clinton administration in 1993. This program demonstrated the effectiveness of light and highly maneuverable kill vehicles that could destroy ballistic missiles in their earliest stage of flight, called the boost phase. This means they knock out missiles just as the missiles enter space and before they release decoys and other payloads designed to confuse or overwhelm the defense. Current systems can't intercept missiles in the boost phase, let alone those deployed in space.

Despite the considerable achievement now at hand, we can't assume the debate over this protection is settled. Opponents of missile defense, including Sen. John F. Kerry, the Democratic candidate for president, continue to insist we stay with the policy that succeeded in preventing missile attack during the Cold War.

But that makes no sense. The enemies have changed. The nature of their leadership is different. Our means of detecting those new threats is unproved. Yet, they say, vulnerability worked then and can continue to work now. But the stakes are too high to rely on this outdated concept of stability. The American people must insist that their government defend them against the current threats. And that means missile defense. Now. No matter who is president.

Baker Spring is the F.M. Kirby research fellow in national security policy at The Heritage Foundation.