A "kill vehicle" hurtled over the Pacific Ocean to intercept and blow up a missile 1,800 miles and 25 minutes into its flight from Alaska to California.

The military's Dec. 5 exercise, designed to simulate a ballistic missile attack by North Korea or Iran, proved to be the latest successful test of the missile defense system the government is developing to protect America.

"It was the largest, most complex test we have ever done," the head of the Missile Defense Agency, Lt. Gen. Patrick O'Reilly, told reporters at the Pentagon afterward. The results, he added, "give us great confidence."

Indeed. The military had shot down test missiles in 37 of 47 previous attempts since 2001.

President-elect Barack Obama doubtless was paying attention. While campaigning, he pledged to protect the nation and its allies from weapons of mass destruction, including those delivered by ballistic missiles.

"In a world with nuclear weapons," candidate Obama said, "America must continue efforts to defend against the mass destruction of its citizens and our allies."

Americans should welcome that commitment. It's certainly in keeping with our nation's 21st-century defense policy, which rejects the Cold War strategy of "Mutual Assured Destruction" -- the theory that no enemy would dare a nuclear strike on the U.S. for fear of massive retaliation.

But Obama hedged support for missile defense, saying he would back a "pragmatic and cost-effective" system that "does not divert resources from other national security priorities until we are positive the technology will protect the American public."

And he broadly warned against the "weaponization" of space.

Once in the White House, President Obama will need to clear up such ambiguities. Poland and other allies are watching. They want to be sure of the new president's commitment to missile defense. And Russia will continue to try to take advantage of uncertainty.

To dispel dangerous doubts, Obama should move quickly on his promise to "spare no effort to protect Americans from the threats posed by nuclear weapons and ballistic missiles." A robust missile defense program should include:

• Dedicating 2 percent to 3 percent of the defense budget. Candidate Obama appropriately called for rigorous testing. Investing in testing is a bargain. Even a limited nuclear missile attack would be catastrophic. It's worth this relatively small investment to be able to shoot such weapons from the sky.
• Consistent development and testing. Testing can't follow traditional acquisition procedures because missile defenses actually are a complex "system of systems." As with a few other weapons systems that first must be built (the Global Positioning System was one), it's not possible to do full operational testing before procurement. Defense officials will have to take incremental steps.
• A "layered" approach. The basic hit-to-kill technology is a success, as seen again Dec. 5. Yet candidate Obama questioned the effectiveness of the program. Perhaps his skepticism stems from viewing elements of the system in isolation. A layered defense -- with boost, midcourse and terminal phases -- makes for an effective defense.
• Fielding space-based elements. Candidate Obama said he wouldn't "weaponize" space, but the missiles he pledged to counter are space weapons. Effective defense against space weapons requires some defense components be mounted in space. An array of 1,000 space-based interceptors and replacements would cost less than $20 billion to build, operate and maintain over 20 years, according to the Independent Working Group, an authoritative panel of experts in related science, technology and national security.
• Cooperating with allies. Candidate Obama confirmed the wisdom of an allied approach. America should continue to cooperate with friends in Asia, Europe and the Middle East. President Obama should implement existing agreements with the Czech Republic and Poland, both NATO allies, to field radar and 10 ground-based interceptors on their soil. By contrast, French President Nicolas Sarkozy's proposed moratorium on missile defenses in Europe undermines NATO solidarity.

If an enemy struck the United States or an ally with ballistic missiles carrying weapons of mass destruction, the world would be forever transformed.

An American president who failed to do the utmost to field a defense against such an attack would face the judgment of history as well as his own people.

Baker Spring is F.M. Kirby Research Fellow in National Security Policy in the Douglas and Sarah Allison Center for Foreign Policy Studies at the Heritage Foundation.