WASHINGTON — Opponents of building a U.S. national missile defense are pointing to the terrifying attacks on the Pentagon and the World Trade Center as evidence that such a system is unnecessary. At the same time, many missile defense supporters say that the Sept. 11 attacks illustrate the need for a national missile defense system.

Missile defense opponents contend that the United States should focus its attention on improving human intelligence gathering rather than building a missile shield to protect against future attacks.

Robbin Laird, an independent defense consultant with offices in France and Washington, said that missile defense "has to go on the back burner."

A missile defense system is too divisive and will not provide any near term benefit to the United States, Laird said. Instead, the United States should focus on more basic issues such as how to protect the Pentagon from terrorist attacks, he said.

Daniel Sheehan, general counsel and director of the Institute for Cooperation in Space, San Francisco, Calif., said that the government should not allow missile defense advocates to use the Sept. 11 attacks to support proceeding with deployment of a missile defense system.

"This is one of the most dramatic examples one could imagine of the lack of effectiveness of spending hundreds of billions of dollars on putting up a huge missile shield," Sheehan said.

But missile defense supporters say that the attacks disprove arguments that missile defense opponents have made for years.

Opponents of a missile defense system have argued that no country will launch a ballistic missile, or permit a terrorist group within its borders to do so, out of fear that the United States would trace the attack and retaliate. However, the United States will surely be able to uncover the identity of those responsible for planning the Sept. 11 attacks and will strike back, missile defense supporters said.

"They surely did this knowing we’d find out who they were, so the argument that no one would use a ballistic missile because we’d know who did it is fallacious," said Henry Cooper, former head of ballistic missile research for the Pentagon.

The attacks did show that ballistic missiles are not the only threat the United States will face, said Cooper, who is currently chairman of the board of High Frontier, a group that supports missile defense. But the United States would be "plain foolish" to turn away from building a missile shield now that it has been hit via another method, he said.

Frank Gaffney, former assistant secretary of defense for international security policy, also said that the attack demonstrated a willingness to attack the United States without fear of repercussions.

"The argument that the Pentagon should only prepare to defend against aircraft, truck bombs, or chemical warfare is just crazy," said Gaffney, who is currently president of the Center for Security Policy here.

Tom Donnelly, deputy executive director of the project for the New American Century here, said that the attacks should serve as a call to U.S. President George W. Bush to accelerate plans for a missile shield.

"Do we need a demonstration of what a large scale attack on an American city can result in?" Donnelly asked.

A missile shield will also be a valuable asset as the United States pursues terrorists around the globe, Donnelly said.

"If we try to get Osama bin Laden, and he’s protected by a state with ballistic missiles, we’d have to be a lot more reluctant" than if the United States already had a national missile defense system, Donnelly said.

Baker Spring, an analyst with the Heritage Foundation here, said the terrorists’ decision to crash airplanes into both towers of the World Trade Center indicates a shift in terrorist motives that further supports the need for a missile shield.

"These guys were not just after terrorizing Americans, they wanted to kill large numbers of people," Spring said. "There was no added value from a terror perspective from running a second plane into the second tower."

This shows that terrorists will try to obtain ballistic missiles so they can inflict increasingly high death tolls in their attacks, Spring said.

Special correspondent Colin Clark contributed to this report.