Almost every aspect of U.S. military policy is likely to be affected by Tuesday’s terrorist attacks in New York and Washington, but one that is certain to come under intense scrutiny is the Bush administration’s plan for a national missile defense (NMD).

In particular, President Bush’s claim that NMD represents the single most important priority for U.S. “homeland” defense is bound to appear highly dubious in light of the apparent success of a still-unidentified terrorist network in causing massive damage to major U.S. institutions and facilities.

As envisioned by the White House, NMD will protect the U.S. against ballistic missile attacks launched by “rogue” states like Iran and North Korea. No such nation currently possesses operational missiles with the range to strike the United States, but the administration claims that one or more of them might acquire such a capability in the next five to 10 years. To defeat such attacks, the Pentagon proposes to spend hundreds of millions of dollars on anti-missile defense systems.

Until Tuesday, the debate over NMD has largely revolved around two questions: First, is the “rogue state” missile threat so severe as to justify the expenditure of hundreds of millions of dollars on missile defenses? Second, can such defenses actually be made to work?

Although the White House insists that the answer to both questions is “yes,” many in Congress and elsewhere have expressed skepticism about these assurances.

Now a new question will emerge: Is NMD really needed when the United States faces a very genuine threat from unconventional forces using improvised weaponry: civilian American aircraft hijacked at American airports? Even if NMD were fully functional and all of its systems worked flawlessly, it would be of no use in defending against attacks of this type.

Given the evident skill and ingenuity of those responsible for Tuesday’s attacks, it is evident that those who are intent on harming the United States can do so quite effectively without relying on ballistic missiles. As many critics of NMD have noted, powerful bombs can be made within the U.S. from commercially available materials, and chemical and biological agents can be smuggled into the country in ordinary luggage. One can also imagine powerful computer viruses and other forms of unconventional attack.

No doubt we will soon see an intense national debate on what measures will be needed to defend against future attacks of this sort. It is probably much too early to speculate on what might be required. But it is very doubtful that rapid construction of NMD will figure among the preferred remedies.

It would be premature at this point for anyone to come forward with a grand blueprint for America’s future defense posture. Much time will be needed to analyze Tuesday’s events and to identify the appropriate protective measures. But given the apparent irrelevancy of missile defenses to America’s actual security situation, one of the first things that Mr. Bush should do as we begin redesigning our defense plans is to put NMD on indefinite hold.