On second thought, however, the attacks unquestionably heightened the fear of terrorism in this country and demonstrated-as far as is now known-that terrorist attacks need not come from "rogue nations" or any nation. To that extent, the devastation of September 11 may have strengthened the political case for missile defense-any defense-against the suddenly demonstrated existence of an international terrorist threat no longer confined to dubious C. I. A. estimates.

The Administration seems to have drawn the latter conclusion, or else its belief in missile defense was so strong as to survive any setback deriving from September 11. In any case, planning and international negotiations for building and deploying a missile defense system seem still to be a high priority in Washington.

What really matters, however, is not the Administation's intent, since that never was in doubt. But does a narrowly divided Congress and the general public share Washington's view that a missile defense still is necessary-or perhaps more needed than ever? Or did the September 11 attacks demonstrate conclusively that terrorists don't need missiles to launch powerful attacks even on the world's only superpower - hence that national missile defense would be a sort of 21st Century Maginot Line?

How Congress and the public view missile defense after Sept. 11 probably can't be known definitively until its primary supporters, President Bush and Defense Secretary Rumsfeld, unveil a specific plan, including some reasonable estimate of its cost and whether or not it can be developed without abrogation of the ABM Treaty with the former Soviet Union.

Whether the Russians, in fact, will make hard and fast agreements on these points remains to be seen. Otherwise, the baleful balance-of-power effects of unilateral U. S. development of a missile defense remain much as they were before Sept. 11. The Chinese, for example, have yet to suggest that they are willing to agree to any such effort, much less to actual deployment of a missile defense whether land, sea or airbased - or all three, as Rumsfeld has suggested.

So the following course of events is not hard to postulate: The U. S. deploys a missile defense, which is also an offensive weapon, much as a man in a sword-fight gains an offensive as well as a defensive advantage if he is given a shield. The Chinese retaliate by building up what is now a relatively limited offensive missile capacity.

That alarms India, a traditional Chinese adversary and also a nuclear power. So, in response to the Chinese buildup, India responds with an increase in its own offensive capacity, or perhaps a missile defense - even one provided by the US. Not only is nuclear proliferation already on the rise, but India's suspicious neighbor, Pakistan, feels threatened by the new Indian firepower. So Pakistan, too, embarks on a buildup of offense or defense or both. And since both India and Pakistan are vital U. S. allies in the war on terrorism, the US can do little to restrain either - in fact, may have to help both maintain a balance against the other, at a higher level of armaments than now exists.

That is speculative, of course-but no more so, and perhaps less, than confident Pentagon and White House predictions that a successful missile defense can be developed and deployed, despite a costly history of failure dating back to the Reagan Administration.