"Times change and we with time"
-Emperor Lothar I (795-855 CE)
In 1837, the British were crushing a rebellion in Upper Canada, now Ontario. The United States, although unwilling to antagonize a superpower by supporting the rebels directly, did not prevent a private militia from being formed in upstate New York. The "volunteers" used a steamboat, the Caroline, to transport arms and men to the rebel headquarters on the Canadian side of the Niagara River. The British responded with a night raid, capturing the vessel as it was docked at Fort Schlosser, New York. They set the boat on fire and sent it over Niagara Falls.
The incident caused disquiet in Washington. British forces, having torched the White House and capitol building in 1814, were again intervening on US territory. Some careful diplomacy followed, with US Secretary of State Daniel Webster conceding that the use of force in selfdefence could be justified when "the necessity of that self-defense is instant, overwhelming, leaving no choice of means, and no moment of deliberation," and provided that nothing "unreasonable or excessive" was done.1 The British accepted Webster's criteria. Over time, as other countries expressed the same view, the Caroline criteria-often referred to simply as "necessity and proportionality"-were transformed into a new right of selfdefence in customary international law.
In 1945, the drafters of the UN charter included self-defence as an exception to their new, general prohibition on the use of force. In addition to the existing customary criteria, three further restrictions were introduced: 1) a state could act in self-defence only if subject to an "armed attack"; 2) acts of self-defence had to be reported immediately to the security council; and 3) the right to respond would terminate as soon as the council took action.
Despite this attempt at a precise definition, the limits of self-defence still depend greatly on customary international law, in part because the charter refers to the "inherent" character of the right. And so, while the right of selfdefence is codified in an almost universally ratified treaty, namely the UN charter, its parameters have evolved gradually-or at least become more easily discernible-as the result of the behaviour of states since 1945.
In 1986, a terrorist attack on a West Berlin nightclub killed one US soldier and wounded 50 more. Many suspected the attack had been sponsored by the Libyan government. Two weeks later, the United States responded by bombing several targets in Libya. Washington claimed the strikes were legally justified acts of self-defence. Yet the claim was widely rejected, with many governments also expressing doubt as to whether the strike met the "necessity and proportionality" criteria for self-defence. The most significant evidence of the lack of support was the refusal of France and Spain-both NATO allies-to allow their airspace to be used by the planes conducting the raid. The widespread negative reactions meant that the legal claim and associated military action did not succeed in changing international law.
About the same time, the additional question arose as to whether the right of self-defence extended to situations where military responses took place on the territory of countries not directly implicated in terrorist acts. In 1985, Israel claimed self-defence when it attacked the headquarters of the Palestine Liberation Organisation in Tunisia. The security council condemned the action, with the United States abstaining rather than voting against (and thus vetoing) the resolution. A number of governments expressed concern that the territory of a sovereign state had been entered to target not the state itself, but alleged terrorists present there.
The law concerning self-defence and terrorism began to change after bombs exploded outside the US embassies in Kenya and Tanzania in 1998. Twelve Americans and almost 300 other people were killed. …