The fires at the Pentagon and World Trade Center were still burning when critics of America's missile-defense program began to argue that the events of Sept. 11 meant President Bush would have to abandon missile defense.

No high-tech shield would have prevented this attack, they said. And assembling a worldwide coalition against terrorism takes precedence, anyway.

"If that means … postponing deployment of a theoretically workable missile defense system against a theoretical 'rogue nation' missile threat, then that is what realism requires of Bush," one columnist wrote.

But if "realism" requires anything, it's that we speed up our missile-defense timetable. The need has been obvious for some time; the events of Sept. 11 merely cement the case.

Think about it. The devastating death and destruction we witnessed in New York and Washington were the result of commercial airliners, loaded with highly combustible jet fuel, crashing into their targets at extremely high speeds. What if the next missile aimed at America's heart carried a nuclear weapon?

Sound far-fetched? No more so than the specter of commercial jetliners destroying the World Trade Center and the creating a huge hole in the Pentagon. Like it or not, what was once unimaginable has become all too imaginable.

If nothing else, the events of Sept. 11 should serve as a somber wake-up call. We no longer can underestimate the capabilities and craziness of those who wish us harm. We now have learned the hard way that defending America means closing down avenues of attack that once would have seemed the stuff of science fiction.

That's why I'm baffled by the continued resistance to missile defense. Rep. John F. Tierney, D-Mass., for example, said the day after the attacks that "this type of incident … is much higher on the list of threats than anything the president would address with this national missile defense program."

But one look at the roster of rogue nations that are actively developing long-range missiles -- including North Korea, Iran and Iraq -- shows why the congressman's confidence is misplaced. We must defend ourselves today against what likely will be the terrorists' weapon of choice tomorrow: ballistic missiles.

In the meantime, of course, we're working to prevent a repeat of the Sept. 11 attacks. We've tightened airport security, we're putting armed sky marshals on commercial flights and we're increasing our intelligence capabilities.

But we shouldn't lose sight of the fact that some of the same regimes that sponsor and support terrorism have aggressive ballistic missile programs. Do we wait until the next tragedy to move aggressively against this threat as well?

Make no mistake. If we don't act now to protect ourselves and our allies from missile attack, we could face an even greater tragedy in the future.

This is no time to pull back on missile defense. It's time to move forward. Not since World War II has America come close to enjoying the broad range of support for its war against terrorism that it does today. Our allies understand that these new threats are unpredictable and that our enemies are their enemies. Now's the time to show the way to a safer world for everyone.

Let's not bet against the resourcefulness of terrorists -- and their supporters in Iran, Iraq and elsewhere -- any longer. Let's be ready next time. That means building a missile defense system. Now.

Edwin Feulner is president of The Heritage Foundation, a Washington-based public policy research institute.