he American government has sounded two main themes in its response to the terrorist onslaught: The United States is going to war against terrorism, and we expect all other countries to support us in the struggle.

The second effort, enlisting the rest of the world, is being done with particular determination and dispatch. Secretary of State Colin Powell began it just the day after the attacks, when he said the administration would make "a worldwide effort to build a coalition against terrorism."

An early and striking success for the effort was the promise by Pakistan's military ruler, Gen. Pervez Musharraf, of "unstinted cooperation in the fight against terrorism." That followed heavy U.S. pressure, with the implicit threat that a failure to cooperate would lead to Pakistan's being treated as a pariah state, cut off from the international finance that it desperately needs.

Pakistan borders Afghanistan, which has harbored the Islamic terrorist who may have directed this week's attacks, Osama bin Laden. Pakistan has been the principal supporter of the fanatic group that now controls most of Afghanistan, the Taliban. It would matter a lot if General Musharraf really changes sides. But will he really be able to, given the influence of Islamic extremists in Pakistan?

To demand forcefully that all nations stand against terrorism is unarguably the right policy. So is the administration's warning that any government harboring terrorists will be treated as itself a terrorist regime.

But war, the other prong of the U.S. response, is a more complicated matter. President Bush was correct to say that "war has been declared on us." But actually taking retaliatory military action would raise serious problems and extremely serious dangers. The enemy in this war is elusive. U.S. intelligence, for example, does not seem to know where Osama bin Laden is. "The American people want action, and they want it now," Representative Bill Young, Republican of Florida, said. But where?

Suppose there were quick strikes by U.S. aircraft on targets in Afghanistan, made to show that we mean business. The result would likely be to kill many impoverished Afghan civilians and few if any terrorists. The danger is that such military action would trigger the Law of Unintended Consequences. That law, as Clyde Haberman of The New York Times put it recently, provides that for every action, there is an excellent chance of producing an opposite and totally disproportionate reaction.

Afghanistan is a prime example. When the Soviet Union invaded there in 1979, the U.S. armed Islamic forces to resist. The country has ended in the hands of anti-Western Islamic extremists.

The danger in the current situation is that hasty, ill-targeted military action could arouse anti-Western sentiments right across the Middle East. That could threaten such important U.S. friends as the governments of Egypt and Jordan — and Saudi Arabia, from which Osama bin Laden is an angry exile and which is at the core of his grievance. He would be delighted at a United States response that destabilized the Saudi regime.

It is crucial, therefore, that the American government act in response to this outrage with calm deliberation. President Bush, inexperienced as he is in war and statecraft, may be tempted to act quickly. But Secretary Powell is a deliberate man, and our military leaders are well aware of the danger of a Middle East backlash.

As we look ahead, it is also plain that we need to improve the physical security of the United States. We have been warned repeatedly of that need but did nothing. Just two years ago an advisory commission on national security headed by former Senators Gary Hart and Warren B. Rudman warned that this country was vulnerable to terrorism and that "Americans will likely die on American soil, possibly in large numbers." Hardly anyone paid any attention.

There is no way to reason with people who think they will go directly to heaven if they kill Americans. (Nor is there a way to reason with the Rev. Jerry Falwell when he says, as he did, that American secularists have offended God and so "helped this happen.") But here on earth, as President Kennedy suggested, we human beings are responsible — and must act to prevent evil.