In Defending Humanity, internationally acclaimed legal scholar George P. Fletcher and Jens David Ohlin, a leading expert on international criminal law, tackle one of the most important and controversial questions of our time: When is war justified? When a nation is attacked, few would deny that it has the right to respond with force. But what about preemptive and preventive wars, or crossing another state's border to stop 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?

In their...