No aspect of national sovereignty or international law is more controversial than the question of the legitimacy of preemptive war for self-defense. When the Bush administration declared preemption an essential pillar of its 2002 National Security Strategy, it provoked legal and diplomatic firestorms that the invasion and occupation of Iraq have only intensified.

But preemption's future as a viable policy option deserves fresh scrutiny, because the threat of unconventional attack requires the threat of unconventional preemption. Traditional military practice constrains both the policy and the legal flexibility to preempt effectively. But unconventional opportunities that increasingly offer policymakers nonlethal, minimally destructive choices to act preemptively are emerging. What the United States needs now is a new doctrine that strikes the best national security balance between the urgent need to preempt hostile acts and the earnest desire to minimize harm. Call it "soft preemption."

For international lawyers and judge advocates general weaned on the Treaty of Westphalia, the U.N. charter and the Geneva Conventions, these next-generation preemption opportunities may look like legal loopholes. In reality, however, they may prove to be a particularly appealing way of adhering to such legally desirable "just war" goals as "proportionality" -- the ethical criterion emphasizing restraint and precision in the use of force.

Suppose, for example, that in the wake of a new wave of suicide bombings in Baghdad, Paris or London, Middle East satellite channels broadcast sermons by imams that praised the attackers as "martyrs" and encouraged further violence. American authorities determine that such broadcasts qualify as "incitement." "Soft preemption" asserts that disrupting or distorting the news channels' satellite transponder signals for 48 or 72 hours could blunt the immediacy of that incitement. The technologies to do this exist. Nonviolent, non-destructive technical interference could arguably save many lives.

Responsible policymakers may disagree over whether such tactics are better than other alternatives, or set destructive precedents for other nations to follow. But it would be irresponsible to deny the value of the option. There needs to be meaningful choices between impotence and violence. Much as the "soft power" of cultural exchange and economic trade complements military "hard power" on the global stage, soft preemption -- as public policy, not just covert action -- is a necessary security counterpart to hard preemption.

Hard preemption -- military strikes -- is almost always condemned as "illegal" and an "overreaction" by the world's nations. Think of Israel's 1981 bombing of Iraq's Osirak nuclear reactor. Similarly, the hard option was seriously considered -- and dismissed -- in the Clinton administration's plans to bomb North Korean nuclear facilities in June 1994. The laws, rules and customs around hard preemption are etched in shades of gray.

In stark contrast to the destructive intent of hard preemption, soft preemption seeks to undermine and compromise an enemy's ability to attack without killing people or blowing up property. Strategically toughening visa and immigration requirements are obvious soft preemption tactics. So are recent policies by European authorities -- notably in France and England -- to deport noncitizens who fund, organize and incite extremist violence.

As the experience of 9/11 and the recent London bombings affirms, coordinating soft preemption strategies would require levels of interagency information-sharing and collaboration that would effectively erase historic organizational distinctions between law enforcement and military defense worldwide. That's particularly true for disrupting the global financial networks that fund terrorism or nuclear proliferation. In the United States, presidential directives empower the secretary of state and the Treasury Department to freeze assets and block financial transactions of terrorists and those supporting them. They also let the United States deny foreign banks access to U.S. markets if they decline to cooperate with American authorities. The United Nations also has counterterrorism conventions targeting finance.

Yet these are essentially law enforcement protocols. If the United States wanted to surreptitiously disrupt the flow of a terrorist funding network by, say, interfering with a bank's computers -- is that a military attack, a law enforcement action, or something else? "Information warfare" lends itself to soft preemption doctrines. However, it defies easy categorization as a weapon subject to the laws of war or criminal law. To the extent that American policymakers should heed the "rule of law," international law at the moment obscures rather than illuminates the appropriate role of soft preemption technologies.

By far the best and most enduring example of soft preemption is economic sanctions. The practice of denying an adversary vital resources that make it easier to wage war or pursue other hostile actions goes back to Athens under Pericles in the 5th century B.C. To send the message that, short of going to war, Athens would punish anyone who challenged her authority, Pericles banned all trade with a city that had aligned itself with Sparta. Alas, those sanctions ultimately led to the Peloponnesian Wars.

During the Cold War, the West set up COCOM -- an international security committee -- to prevent the Soviet Union and its Warsaw Pact allies, along with China, from getting access to commercial high-tech "dual-use" technology that could be used in weapons systems. It enjoyed mixed success. Most recently, a good case could be made that Western sanctions against Iraq were far more successful than most analysts had anticipated. Iraq's dilapidated economy and military infrastructure contributed to its rapid collapse in the Persian Gulf War.

Sanctions can indeed undermine the ability of an adversary to engage in certain large-scale conflicts. But their preemptive national security impact has been obscured by their use to punish nations for misbehavior -- i.e., South Africa and Serbia -- instead of to undermine their ability to prosecute a war. Moreover, economic sanctions tend to target and punish entire populations rather than specific regimes.

The history of economic sanctions offers unhappy insight into the future of soft preemption. Sanctions are processes, not events. Some international lawyers argue that attempts to "add teeth to" or actually enforce sanctions -- by means of blockade or interdiction, for instance -- are themselves acts of war that make retaliation legitimate. Others sharply disagree.

In truth, soft preemption for national self-defense inspires ambivalence from the international legal and diplomatic communities. Consider the Israeli wall. In a controversial hearing, the World Court decided that the properties seized to build the wall and the burdens imposed upon the local population outweighed Israel's legal right to build this barrier in self-defense. International law is similarly conflicted about the introduction of nonlethal weaponry; it forbids the use of certain weapons -- tear gas, for example -- on foreign troops that police forces routinely use on civilians. The rise of soft preemption options threatens to make international law an even less useful guide for national security policymakers worldwide.

There are undeniably provocative aspects to the soft preemption doctrine, as there are to any self-defense approach. If Venezuela disrupted BBC, Telemundo or CNN satellite broadcasts throughout Latin America to prevent "incitement" of a coup against President Hugo Chavez, for example, that would surely escalate regional tensions. If China launched denial-of-service Internet attacks against American institutions that fund supporters of "Free Tibet" or encourage Taiwanese nationalists, the international community would confront a real challenge.

But the ability to preempt softly doesn't inherently invite malicious global mischief any more than the ability to launch a cruise missile guarantees its firing. Responsible nations would have every reason to think twice about soft preemptions in self-defense; irresponsible ones could have bigger problems than soft preemption to deal with if they're not careful. Anybody who engages in soft preemption must be prepared to escalate to the harder stuff if necessary.

The United States would do the world a service if it declared that, as technology permits a broader array of less violent, less destructive interventions to undermine the abilities of non-state actors and their state sponsors to strike, it will exercise those soft options to deter and disrupt any attacks. Collateral damage to such interventions is preferable to the collateral damage of bombs and bullets gone astray. Soft preemption is soft power with a vengeance, and a public policy option that deserves a good hard look.

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Michael Schrage, a senior adviser to MIT's security studies program, occasionally does non-classified work with the defense community.