Times change and we with time-Emperor Lothar I (795-855 CE)In 1837, British were crushing a rebellion in Upper Canada, now Ontario. The United States, although unwilling to antagonize a superpower by supporting rebels directly, did not prevent a private militia from being formed in upstate New York. The volunteers used a steamboat, Caroline, to transport arms and men to rebel headquarters on Canadian side of Niagara River. The British responded with a night raid, capturing vessel as it was docked at Fort Schlosser, New York. They set boat on fire and sent it over Niagara Falls.The incident caused disquiet in Washington. British forces, having torched 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 use of force in selfdefence could be justified when the 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 same view, Caroline criteria-often referred to simply as necessity and proportionality-were transformed into a new right of selfdefence in customary international law.In 1945, drafters of UN charter included self-defence as an exception to their new, general prohibition on use of force. In addition to 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 security council; and 3) right to respond would terminate as soon as council took action.Despite this attempt at a precise definition, limits of self-defence still depend greatly on customary international law, in part because charter refers to inherent character of right. And so, while right of selfdefence is codified in an almost universally ratified treaty, namely UN charter, its parameters have evolved gradually-or at least become more easily discernible-as result of 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 attack had been sponsored by Libyan government. Two weeks later, United States responded by bombing several targets in Libya. Washington claimed strikes were legally justified acts of self-defence. Yet claim was widely rejected, with many governments also expressing doubt as to whether strike met necessity and proportionality criteria for self-defence. The most significant evidence of lack of support was refusal of France and Spain-both NATO allies-to allow their airspace to be used by planes conducting raid. The widespread negative reactions meant that legal claim and associated military action did not succeed in changing international law.About same time, additional question arose as to whether right of self-defence extended to situations where military responses took place on territory of countries not directly implicated in terrorist acts. In 1985, Israel claimed self-defence when it attacked headquarters of Palestine Liberation Organisation in Tunisia. The security council condemned action, with United States abstaining rather than voting against (and thus vetoing) resolution. A number of governments expressed concern that territory of a sovereign state had been entered to target not state itself, but alleged terrorists present there.The law concerning self-defence and terrorism began to change after bombs exploded outside US embassies in Kenya and Tanzania in 1998. Twelve Americans and almost 300 other people were killed. …