With the "new" century now 8 years old (and counting), it's time to finally shelve some expired ideas left over from the last years of the 20th century.

Every facet of Cold War "common sense" that argued against missile defense makes no sense in the 21st century. deterrence (threatening to lob atomic warheads at whoever might attack you as a way to prevent being attacked in the first place) just doesn't work anymore.

For one thing, nascent nuclear nations -- like North Korea -- may not understand the "rules" of Cold War-style deterrence and could well blunder into nuclear conflict. For another, rogue states and non-state actors may not be deterred by the U.S. nuclear arsenal.

On the other hand, an effective defense against ballistic missiles is not only achievable, but desirable. In a dangerous world, strategic defense will make America more -- not less -- safe. In short, there's no rational reason to intentionally leave the United States vulnerable to ballistic missile attack.

But rather than accept that fact, opponents of missile defense are just inventing new arguments, even if they don't make much sense.

"Missile defense won't stop nuclear terrorism," Retired Gen. Robert Gard railed recently on the Huffington Post. Gard is correct. But that's a pathetic complaint against building missile defenses.

First of all, it makes little sense to focus on one threat to the exclusion of others. America should be able to accomplish the national security equivalent of walking and chewing gum at one time. We can defend against more than one danger.

Gard's assertion that any American enemy would obviously chose to employ a "suitcase" bomb or a "dirty" bomb also bears some analysis. It is worth pointing out, for example, that nobody has ever built a suitcase bomb.

The United States and Russia built some "man-portable" nuclear weapons. (I should know; as an Army second lieutenant, I lugged around the American version.) The U.S., for sure, got rid of its supply. The Russians claim they've done likewise. Stories of "loose" Russian nukes may persist, but none have ever turned up.

In short, it's unlikely there are many terrorists out there with the smarts to put a bomb in suitcase -- and it's unlikely there are any to buy, either.

Sure, terrorists could build a "moveable" nuclear bomb and stick it on a boat or plane and send it to America without a return address. But while this sounds like a cool idea for a Tom Clancy novel, in reality it would be a lot harder to accomplish. Certainly, no semi-intelligent terrorist would put a bomb in a shipping container and send it to New York. Containers, after all, are routinely lost, pilfered, crushed or otherwise waylaid.

Moving a nuclear weapon would likely require 100 percent success "guaranteed." And that would require a sophisticated smuggling operation -- one far more effective than drug and arms smugglers routinely use. Criminal smugglers expect to lose some of their product along the way (so they ship more to keep the profit margin up). Nuclear smugglers, by contrast, couldn't afford even one mistake.

Nuclear smuggling is also a lot harder than it used to be because of the Bush administration's Proliferation Security Initiative. PSI is a multinational effort to thwart trafficking in materials, technology and actual weapons. It poses a real problem to anyone interested in FEDEXing nuclear weapons or trying to ship them themselves.

Smuggled weapons are also not as effective. The danger of "dirty bombs" (explosives that simply spread radioactive material) is, excuse the pun, way overblown. A truck-borne small "real" nuclear weapon detonated in downtown New York might kill 40,000. The same weapon detonated as the warhead of a missile in a low-altitude airburst might cause half-a-million causalities. If you wanted to send a message to America, which attack mode would you chose?

And missile threats are not out of reach for terrorists with even modest means. Short-range ballistic missiles can be bought on the open market. They can be launched at ships from sea that never see an American shore or come near a Coast Guard cutter.

The threat of ballistic missiles is real and likely to grow in the future unless our government acts. PSI will help, but missile defense is an insurance policy. And as Gen. Trey Obering, former commander of the Missile Defense Agency, has observed, "It's better than insurance, because insurance only pays off after the fact."

Missile defense must be an important part of "providing for the common defense." If America does not act, its enemies will.

James Jay Carafano is a senior fellow at The Heritage Foundation and the author of GI Ingenuity: Improvisation, technology and Winning World War II.