HONOLULU - Suppose, for the sake of mental exercise, the Japanese flotilla bearing down on Hawaii from the north Pacific 61 years ago had been discovered before it launched its dive-bombers and torpedo planes to attack the U.S. fleet at Pearl Harbor. What should the Americans have done?

U.S. political and military leaders could have decided to do nothing until the Japanese had actually struck.

Or Washington could have tried to deter the Japanese by demanding that Tokyo order its fleet to turn around, and U.S. military commanders could have steamed their fleet into a blocking position between the Japanese warships and Pearl Harbor.

Or the Americans, sensing a clear and present danger, could have ordered a pre-emptive strike in hopes of stopping the Japanese attack, or at least blunting it.

As it was, the Japanese surprise attack took the lives of about 2,400 Americans and nearly crippled the fleet. The sunken battleship Arizona, in which 1,177 died, still leaks oil in mute testimony to America's greatest naval disaster.

This speculation is pertinent to a controversy raging in the United States and Asia today - the doctrine of pre-emptive war. President Bush triggered it in June at the U.S. Military Academy at West Point, saying that Americans should "be ready for pre-emptive action when necessary to defend our liberty and to defend our lives." Pre-emption was codified in Mr. Bush's national security strategy in September as a cornerstone in the war on terror and "rogue states" that hate America.

Critics quickly leapt to condemn it. Former Vice President Al Gore, Mr. Bush's opponent in the 2000 election, asserted that the doctrine would destroy "a world in which states consider themselves subject to law." But Sen. Edward M. Kennedy of Massachusetts, another leading Democrat, acknowledged that "no nation should have to suffer a certain first strike before it has the legitimacy to respond."

The ruckus spread to Asia last week when Prime Minister John Howard of Australia contended that his nation would have the right to mount a pre-emptive strike if threatened with a conventional or terrorist attack and "there was no alternative."About 100 Australians were killed in the terrorist bombing in Bali, Indonesia, in October.

Leaders in Indonesia, Malaysia, Thailand and the Philippines were indignant about what they declared would be a violation of their sovereignty.

Prime Minister Mahathir Mohamad of Malaysia, who has crossed verbal swords with Australian leaders before, charged Mr. Howard with being "insensitive" and "arrogant."

None of those leaders, however, assured Australia that it would not be attacked from their territory.

Part of the argument appears to arise from a failure to draw distinctions between pre-emption and prevention. A pre-emptive strike occurs when a nation has convincing evidence that it is about to be struck and thus hits first. Preventive war or surprise attack, such as Japan's assault at Pearl Harbor, is intended to strike an unsuspecting target.

Pre-emptive strikes are as old as war. The Chinese strategist Sun Tzu wrote 2,500 years ago: "Forestall your opponent by seizing what he holds dear." Thomas Jefferson justified pre-emptive war in the Declaration of Independence, writing that the British intent to impose absolute despotism gave Americans the right and duty "to throw off such government." George Washington led his troops across the Delaware River to strike Hessian mercenaries before they could attack his forces.

More recently, Israel mounted a pre-emptive strike against Egypt in 1967 when it mobilized military forces on Israel's borders. In 1981, Israel launched an aerial pre-emptive strike to destroy an Iraqi nuclear reactor at Osirik, near Baghdad, before it could churn out nuclear weapons.

In 1998, U.S. and South Korean military leaders included pre-emptive strikes in their revised war plans. If the North Koreans showed unmistakable signs of preparing to strike, and then-President Bill Clinton decided not to wait until South Korea had been attacked, U.S. forces had targets in North Korea already picked out and weapons assigned to destroy them.

The critical issue in justifying pre-emptive action is convincing evidence of clear and present danger.

Americans today seem to have accepted the premise of an imminent threat. A fresh Gallup Poll reports that nine in 10 believe Iraq has weapons of mass destruction or is trying to develop them. Among those of that view, four in five believe Saddam Hussein would use them against the United States.

Richard Halloran is a journalist and a writer. He lives in Honolulu.