When President Bush says he has committed America to a "just war," he invokes a concept that has troubled philosophers for centuries: Under what circumstances can war be justified?

True pacifists would say war is never an option, but many thinkers have concluded that there are times when war may be better than its absence. That idea originated with St. Augustine, a Christian philosopher who died in 430 A.D.

Since then these criteria for deciding whether a war is morally justifiable have won general agreement:

Just cause. The war must confront an unquestioned danger. War fought in self-defense is the classic example.

Competent authority. The leader committing a nation to war must be acting on behalf of his people.

Right intention. The reasons set forth must be the actual objectives. Thus some theologians have said Bush would be justified in waging war to force Iraq to give up its conquest of Kuwait, but not to destroy Saddam Hussein's capacity to make war.

Last resort. All peaceful alternatives must have been exhausted.

Probability of success. The chances of achieving the war's purpose must be reasonably assured of success.

Proportionality. The good that will be achieved by war cannot be outweighed by the harm.

Theologians and philosophers are not agreed on whether the war against Iraq meets the tests.

Bush, addressing the National Religious Broadcasters Association on Monday, said it does. "It is a just war and it is a war in which good will prevail," he said. "Our cause could not be more noble."

Despite the agreed-upon criteria for measuring war, "there's no acid test," said R. Paul Churchill, a professor at George Washington University and a member of Concerned Philosophers for Peace, which debates the issue routinely.

"The phrase 'just war' is very ambiguous and causes a lot of confusion," added Douglas Lackey, author of "The Ethics of War and Peace" and a professor at Baruch College of the City University of New York. "In the weak sense, it means that war is permissible, it is not contrary to morality to fight the war. But, in the stronger sense, it means the war is obligatory, it is your moral duty to fight it."

A few days before the bombs started falling, the Society of Christian Ethics decided in a 97 to 20 vote that the use of violence in the Middle East did not meet just-war criteria.

And Archbishop Daniel E. Pilarczyk, president of the National Conference of Catholic Bishops, said going to war "would likely violate the principles of last resort and proportionality."

Last week, Cardinal Bernard Law of Boston became the highest-ranking Roman Catholic prelate to support Bush's decision.

"Even as we echo the prayer of Pope Paul VI, 'No more war, war never again,' with heavy hearts we realize that such a prayer is not fulfilled at the price of granting tyrants and aggressors an open field to achieve unjust ends," the cardinal wrote.

The Rev. Billy Graham said, "There comes a time when we must fight for peace."

Lackey said everyone who ever made war thought his war was just. "The belt buckles of German soldiers in World War I were embossed, 'Gott mit uns' -- 'God is with us.' "