Legalities aside, Boston University's Andrew J. Bacevich warns that US first strikes will "lower the bar" for pre-emption and give other nations--India? China? Israel?--new excuses for aggression.

On the far left, pre-emption angst runs high. "What was once the frothing of right-wing ideologues is now on the verge of becoming national policy," sputters The Nation. Even a few Republicans express unease; Nebraska Sen. Chuck Hagel notes "a dangerous arrogance and a sort of 'Pax Americana' vision."

As can be seen, the subject of pre-emption lends itself to hyperbole. We're about as close to having a New Imperium or Pax Americana as we are to having a dictatorship of the proletariat, but anguished and angry warnings keep coming.

Stirring the controversy is President Bush's "National Security Strategy of the United States," a 31-page paper made public Sept. 20. It asserts a right to forcibly disarm a state whose nuclear, chemical, or biological weapons could threaten us or our allies, or wind up in the hands of terrorists. It states explicitly that the US is not bound to wait to be attacked, but may choose to hit first.

Obviously, Iraq is the first test. It won't be the last, so it's worth considering what Bush's new doctrine does and does not mean.

The Sept. 11 attacks changed US strategy, but not all at once. Homeland security rose in priority. The President declared the "Bush doctrine," threatening to use force not only against terrorists but also against their state sponsors. Pre-emption is the latest piece of the strategic puzzle to fall into place.

This step reflects three realities, say officials. First, stateless terrorists can't be deterred. Second, there is an ever-present danger that terrorists could acquire weapons of mass murder from rogue nations. Third, terrorists using those weapons could kill millions.

The paper states: "The inability to deter a potential attacker, the immediacy of today's threats, and the magnitude of potential harm ... [mean] we cannot let our enemies strike first." In sports parlance, that means the best defense is a good offense.

This is a clear statement, but it wasn't long before it had become enveloped in a fog of claims and counterclaims.

• Claim: US pre-emption is radically new and unprecedented. When it comes to pre-emptive action, however, the US is hardly a virgin. It has engaged in such activity for decades. In 1962, President Kennedy imposed a "quarantine" on Cuba, though it had not struck the United States. Operations in the Dominican Republic in 1965 and Grenada in 1983 were meant to thwart Communist activity. Attacks on Panama in 1989 and Serbia in 1999 were unprovoked in any direct military sense. This list is not exhaustive.
• Claim: It is illegal to use armed force until attacked. The right to self-defense is universally recognized, and self-defense doesn't have to be a post-attack event. The Congressional Research Service says pre-emption--an attack to thwart a clear, imminent threat--"has a home" in international law. Example: In 1967, Israel attacked first and defeated a host of Arab forces drawn up for battle. Few contend that Israel wasn't justified in doing so.
• Claim: "Pre-emption" is legal, but "prevention" is not, and prevention is what Bush wants. This matter turns on a threat's "imminence." Israel's 1967 attack was pre-emption because foes were ready to strike. Japan's 1941 Pearl Harbor attack was preventive, and thus morally wrong; America was not a threat, and Japan should have waited to see if it would become one. Bush officials see this as a distinction without a difference in today's world. Terrorists can use weapons of mass murder without giving any warning at all. "Prevention" is the only workable form of "pre-emption."
• Claim: The US will set a dangerous example around the world. Alan J. Kuperman of Johns Hopkins University says US policies have exerted a restraining influence on India, which has thought about attacking Pakistan's nuclear force. Others claim Pakistan, China, Israel, and others might feel freer to act militarily. However, Bush officials note that none of these countries have been noticeably deterred from using military force when they feel their national security is at stake.

Adoption of pre-emption as a declared option marks an evolutionary, not revolutionary, change in US strategy. Time-tested concepts of deterrence and containment--which themselves sparked fierce criticism--are still available to a President.

The embrace of military pre-emption brings risks, no question. Washington must make sure it strikes the right target, even though intelligence is imperfect. There is a near-absolute requirement to succeed, because there will be no second chance.

These, however, are practical questions. The key theoretical question has been asked--and answered--by defense official Kenneth L. Adelman: "Who among us would not have attacked Osama bin Laden on Sept. 10, 2001? Is there any argument for not doing that? I can't see it in my wildest dream."