Americans will remember the events of Sept. 11 as a turning point in their history, and many have begun to wonder about the legal as well as the moral aspects of the coming response. In his statement to the nation Sept. 20, President Bush outlined the American response but did not mention the role of international law. As a lecturer in international law and retired judge advocate of the Marine Corps, I find a clear, legal path for American military action.

International law is not widely understood. A commentator on National Public Radio recently opposed any military response to the attack and argued that the perpetrators should be brought before the World Court. But the International Court of Justice can hear only matters between nations, and the International Criminal Court (ICC) is years away from effective operation. Even when it begins, it is doubtful that the ICC will have jurisdiction over terrorism, which still lacks a commonly accepted international definition.

In the short term, the United States is coordinating a wide array of measures — diplomatic, economic and informational — while attempting to form an international coalition against terrorism. American leadership is preparing the nation for a long-term campaign. At some point, we can expect military force to be part of the response.

International law provides some guidance as President Bush formulates an appropriate strategy. Article 2(4) of the United Nations Charter prohibits members of the United Nations from taking forcible action against the territorial integrity and political independence of other states, unless authorized by the Security Council. But Article 51 of the charter recognizes the inherent right of self defense, and the United States has relied on that provision to justify other forcible responses to terrorism, such as Operation Eldorado Canyon in Libya in 1988 and the cruise missile attacks in Afghanistan and Sudan in August 1998.

There are essentially two questions. First, is a forcible response justified? The coordinated hijacking of four aircraft and the killing of thousands of civilians on American shores must certainly be considered an "armed attack" under Article 51 of the U.N. Charter. As part of that question, what level of proof is required that Osama bin Laden, or another individual, is responsible for these actions.

Without direct evidence, such as an admission by Bin Laden or record of his conversation, it may not be possible to achieve American courtroom standards of proof. Bin Laden is said to control a decentralized structure of terrorists; he may in fact have been ignorant of the exact plans of the attackers. And there is no judge to carefully weigh the evidence and apply traditional legal principles of "beyond a reasonable doubt" or "preponderance of the evidence." President Bush will have to make the decision, based upon the law, common sense and the U.S. national interest.

The real question is whether evidence of openly inciting violence against American civilians, as well as training and paying for individuals to carry out those acts, is enough proof.

The statute of the International Criminal Court, Article 25, provides that a person is criminally responsible if he "orders, solicits or induces" the crime or "assists in the commission of the crime" by "providing the means for its commission." Using this standard, our leaders may already have enough evidence to justify the use of deadly force against bin Laden and his network. Finding him and applying the force will be difficult missions.

The second question is this: If force is justified, against whom can the action be taken? Certainly the perpetrators can be targeted, but what of states that support them? The Taliban government of Afghanistan has harbored Osama bin Laden and tolerated the existence of his training camps, despite his public calls to kill Americans.

Legal scholars disagree on how much support is enough to justify a reprisal against a state. But in my judgment, a state's general and continuing support for a group known to be engaged in terrorism is enough to establish state responsibility for aiding or conspiring in the act itself. In the case of the Taliban government of Afghanistan, there appears to be enough evidence to justify the use of force against state assets, such as airfields, military bases or facilities directly tied to the support of terrorism.

Legal scholars tend to agree that if a forcible response to a terrorist act is made, it must be timely, proportionate and discriminate. No responsible American would approve if the United States leveled an Afghan city, or made indiscriminate attacks against civilians in Afghanistan or anywhere else. But if the government of Afghanistan is knowingly harboring an individual or organization engaged in terrorism against the United States, then it becomes a lawful target for U.S. military force.

This conclusion raises other problems, including the fact that Afghanistan has very few traditional military targets, and there is always the risk of civilian and U.S. military casualties. More important, the use of force, though lawful, could cause an escalation in the Islamic world and inflame an already dangerous situation.

International law is in a constant state of evolution. If President Bush chooses the military option in Afghanistan, support can be found under international law and the concept of self-defense. For the first war of the 21st century, the choices will not be easy.

To paraphrase President Bush in his statement to the nation on Sept. 20, this may be the time to bring justice to our enemies.

Frederick Michael Lorenz lectures at the University of Washington Jackson School of International Studies and is an adjunct professor of law at Seattle University. He retired from the Marine Corps in 1998 after a 27-year career as a judge advocate.