In a new book, Cornell Law School faculty member Jens David Ohlin asks -- and answers -- one of the most debated questions of our time: When is war justified?

In the book "Defending Humanity: When Force Is Justified and Why" (Oxford University Press, 2008), Ohlin, a leading expert on international criminal law, and co-author George Fletcher, a law professor at Columbia University, argue that a nation has the right to defend itself when attacked. But what about a pre-emptive war? Or crossing a country's border to stop a genocide? Was Israel justified in initiating the Six Day War, and was NATO's intervention in Kosovo legal? What about the U.S. invasion of Iraq?

An assistant professor of law, Ohlin offers an innovative theory on the legality of war, with clear guidelines for evaluating these types of interventions. The authors compare self-defense in domestic and international law and the differences between French and English-language concepts of self defense -- and find solid legal grounds for countries to intervene with force to protect themselves and others.

Much of the confusion on the subject, say the authors, stems from a persistent misunderstanding of Article 51 of the United Nations Charter, which appears to be very clear on the use of military force: It is only allowed when authorized by the Security Council or in self-defense. The authors argue that this has led to justifying force when the Security Council refuses to act or when self-defense is thought not to apply -- and to the difficult dilemma of declaring such interventions illegal or ignoring the U.N. Charter altogether.

Fletcher and Ohlin discuss going back to the domestic criminal law concepts upon which the U.N. Charter was originally based, in particular, the concept of "legitimate defense," which encompasses not only self-defense but defense of others. They argue that the concept of legitimate defense gives political leaders, courts and scholars a solid basis under international law for states to intervene with force -- not just to protect themselves against an imminent attack but also to defend other national groups. However, the authors say, this concept is lost in the English-language version of the charter, but that it is a vital part of the French and other non-English versions. In addition to providing a new legal argument for "humanitarian interventions," the authors' theory also places strict limits on when this doctrine can be evoked to justify a war. Chapters include analyses of aggressive war, humanitarian intervention and the Bush doctrine of preventive war.

Ohlin's scholarly work has appeared in the Columbia Law Review, the American Journal of International Law, the Journal of Criminal Law and Criminology, and the Journal of International Criminal Justice.