With Russian President Dmitry Medvedev issuing threats about America's planned missile defense for Europe, it may be time to remind the Obama administration why we need such defenses in the first place -- in Europe and elsewhere.

Plainly put, we live in a dangerous world, and we need to do everything possible to remain safe.

Iran and North Korea are trying to acquire the ability to target us and our allies with ballistic missiles tipped with nuclear weapons. And they're not alone: North Korea has provided such technology to other hostile nations, including Syria.Think the Middle East is a powder keg now? Imagine a state like Syria equipped with nuclear weapons and the means to deliver them. Hello, World War III.

President Obama is still wed to his "reset" strategy with Russia. But the fact that our efforts to shield ourselves from catastrophic attack are upsetting Moscow pales next to the folly of remaining needlessly vulnerable. So even as we work to keep rogue states from obtaining the ultimate weapon, we can't neglect the need to protect ourselves in a worst-case scenario.

Indeed, a missile shield could do much to prevent the Irans and North Koreas of the world from acquiring such weapons. After all, why go to the trouble and expense of building them if you know they're unlikely to succeed? It wouldn't be practical. So missile defense, besides bolstering our security considerably, can help keep the world from becoming a more volatile place.Some critics may still insist that missile defense isn't technologically feasible. Actually, the science has advanced to the point where this argument doesn't hold water. Test after test has shown that you can, in fact, "hit a bullet with a bullet." And if you couldn't, why would our adversaries be so dead-set on stopping us? Why not sit back and let us pursue a pipe dream? Because they know what the critics don't: missile defense works. And it means they won't be gaining the upper hand.Besides, in a post-9/11 world, it's irresponsible to rely only on deterrence anymore. During the Cold War, you could get away with "Mutually Assured Destruction." Neither the United States nor the Soviet Union was about to launch an attack guaranteed to invite major retaliation. But with states with unpredictable leaders and terrorist camps racing to become nuclear powers, such a policy would be more "MAD" than ever.

As Baker Spring and Michaela Bendikova explain in a recent Heritage Foundation paper, we need a three-step plan to ensure we get a missile defense able to do the job:

1) Improve the Navy's Aegis-based missile defense system. Here we'd be building on working technology -- technology that has already proven itself in the field on the Navy's Aegis ships. The system has been modified so that it can shoot down short-range to intermediate-range ballistic missiles and detect and track ballistic missiles of all ranges. The next step is to adjust it so it can shoot down long-range ballistic missiles in the late "midcourse" stage of flight.

2) Build a layered missile defense. We need a network with land, sea, air and space capabilities. That means locating sensors throughout the world and in space. It also means we have to increase the number of interceptors we have to counter long-range missiles. With a layered system, our chances of destroying an incoming missile are greatly increased.

3. Develop space-based interceptors. "All but the very shortest-range ballistic missiles travel through space," Spring and Bendikova write. "Thus, the most capable missile defense system would locate interceptors where the missiles would fly -- in space."

There's no excuse to delay or shortchange our ability to defend ourselves as fully as possible. We need an immediate and comprehensive missile defense. With it, we can deprive our enemies of a powerful weapon. What are we waiting for?

Ed Feulner is president of The Heritage Foundation (www.heritage.org).