On October 7,2001, the United States and the United Kingdom launched operation Enduring Freedom. Enduring Freedom was a massive aerial and land operation on the territory of Afghanistan in response to the September 11 terror attacks on the United States. The two governments justified Enduring Freedom as an exercise of lawful self-defense. This article examines the elements of self-defense, applying them to Enduring Freedom. At the outset, Enduring Freedom did indeed meet the conditions of lawful self-defense, but later stages of the operation may have gone beyond the bounds of proportionality. The article also looks at the alternatives to self-defense for those cases that do not meet the threshold for all-out military action on the territory of a foreign state.

The right to take military action on the territory of another state is at the core of a self-defense and why it is of concern to international society. Governments have decided they cannot eliminate the right to use force in self-defense in all cases. Nevertheless, they have, through international law, limited force in self-defense to the most exigent circumstances. Unless a state has received United Nations Security Council authorization, it must meet four conditions to engage in lawful self-defense: First, the defending state must be the victim of a significant armed attack. Second, the armed attack must be either underway or the victim of an attack must have at least clear and convincing evidence that more attacks are planned. Third, the defending state's target must be responsible for the significant armed attack in progress or planned. Fourth, the force used by the defending state must be necessary for the purpose of defense and it must be proportional to the injury threatened.