You know, build a shield that would protect the United States and its allies and -- somehow, someway -- keep the 1972 Anti Ballistic Missile (ABM) Treaty, an agreement that bars the deployment of a national (let alone global) missile defense?

Quite a few missile-defense opponents, those who stoutly maintain the treaty is the "cornerstone of strategic security," think so. And lately, even some missile-defense supporters have started to agree with them.

Fortunately, President Bush knows better. "We must move beyond the constraints of the 30-year-old ABM Treaty," he said in his recent address at the National Defense University. "No treaty that prevents us from addressing today's threats, that prohibits us from pursuing promising technologies to defend ourselves, our friends and our allies, is in our interests."

Opponents are unmoved. Move forward on missile defense, they say, but remain ambiguous about the treaty. They argue that if the administration takes a "go-slow" approach to the treaty, Russia and America's allies will be more inclined to go along with missile defense.

Nothing could be further from the truth. In fact, such an approach to resolving the treaty issue could wind up making it harder to jettison.

Today, Russia has scaled back its criticism of a missile defense considerably. It has shown its true hand -- that it's willing to cut more nuclear warheads in exchange for cooperation on missile defense. This shift came because of President Bush's steady, determined advocacy, not a reliance on "arms control."

The same holds true for our European allies. They look to us for leadership and partnership on security issues, but the United States can't be an effective leader or partner while displaying a lack of commitment. President Bush campaigned on a promise to protect Americans from missile attack. His persistence since the election has led America's allies to begin showing some support for the idea -- or at least stop ruling it out.

But we can't get closer to deploying a missile defense without taking the next step: scrapping the ABM Treaty. The treaty is incompatible with missile defense because it prohibits the testing and development -- not to mention the deployment -- of an effective missile shield. For the president to even entertain the notion of retaining the treaty would be tantamount to backing off his pledge to deploy a missile defense as soon as possible.

Besides, in the opinion of some experts in international law, the treaty is no longer legally binding. Recall that our fellow signer, the Soviet Union, dissolved in 1991, leaving us without a treaty partner.

Unless and until the United States formally abandons the treaty, its commitment to actually deploying a system remains questionable. This is precisely what occurred during the Clinton administration. By pandering to a defunct agreement, Clinton proved he was never really committed to deploying effective missile defenses. As a result, the response from Russia and our allies remained highly negative.

When did they start to show signs of support for missile defense? Practically from the day President Bush was elected. This is because they were convinced of his commitment. But if the president chooses to be vague about the treaty's status, it will signal that perhaps he isn't so committed after all. And if they question his commitment to deploying missile defense in the near term, they likely will revert to the tactics they used to keep missile defense under wraps during the Clinton administration.

Once President Bush kills the ABM Treaty once and for all, he will have set the bar for the debate. As long as the treaty exists, public debate will center on how to amend it. But with it promptly placed on the ash heap of history, the debate will shift to how to develop the most effective defense against ballistic missiles -- and bring U.S. and Russian nuclear warhead numbers more in line with the modern era.

Presidents can't afford to make rash choices, but being clear about American intentions and abandoning the ABM Treaty is the kind of leadership and partnership America's allies want, its friends rely on, and its potential adversaries need to understand.

Jack Spencer is a policy analyst for defense and national security in the Davis Institute for International Studies at The Heritage Foundation.