Caroline affair The Caroline " affair ( also known as the Caroline " case ) was a series of events beginning in 1837 that strained relations between the United States and Britain . Events . A group of Canadian rebels , led by William Lyon Mackenzie , seeking a Canadian republic , had been forced to flee to the United States after leading the failed Upper Canada Rebellion in Upper Canada ( now Ontario ) . They took refuge on Navy Island on the Canadian side of the Niagara River , which separates the two countries ( between Ontario and New York ) and declared themselves the Republic of Canada under MacKenzie ' s " general " Rensselaer Van Rensselaer ( nephew of General Stephen Van Rensselaer ) . American sympathizers supplied them with money , provisions , and arms via the steamboat SS " Caroline " . On December 29 , Canadian loyalist Colonel Sir Allan MacNab and Captain Andrew Drew of the Royal Navy commanding a party of militia , acting on information and guidance from Alexander McLeod that the vessel belonged to Mackenzie , crossed the international boundary and seized the " Caroline " , chased off the crew , towed her into the current , set her afire , and cast her adrift over Niagara Falls , after killing one black American named Amos Durfee in the process . His body was later exhibited in front of a recruiting tavern in Buffalo , New York . US newspapers falsely reported " the death of twenty - two of her crew " when in fact , only Durfee was killed . Public opinion across the United States was outraged against the British . President Martin Van Buren protested strongly to London , but was ignored . On May 29 , 1838 , 13 raiders , mostly Canadian refugees from the 1837 rebellion , led by William " Pirate Bill " Johnston , retaliated by capturing , looting , and burning the British steamer " Sir Robert Peel " while she was in U . S . waters . President Martin Van Buren sent General Winfield Scott to prevent further American incursions into Canada . However , there were several other attacks , the biggest being the Battle of the Windmill in November 1838 . Later that year , Irish - Canadian rebel Benjamin Lett murdered a loyalist , Captain Edgeworth Ussher , who had been involved in the incident . The case was finally disposed of by U . S . Secretary of State Daniel Webster and Alexander Baring , 1st Baron Ashburton , in the course of their negotiations leading to the Webster – Ashburton Treaty of 1842 . Secretary Webster admitted that the employment of force might have been justified by the necessity of self - defence , but denied that such necessity existed , while Lord Ashburton , although he maintained that the circumstances afforded excuse for what was done , apologized for the invasion of United States territory . Anticipatory self - defense . This incident has been used to establish the principle of " anticipatory self - defense " in international politics , which holds that it may be justified only in cases in which the " necessity of that self - defense is instant , overwhelming , and leaving no choice of means , and no moment for deliberation " . This formulation is part of the " Caroline " test . The " Caroline " affair is also now invoked frequently in the course of the dispute around preemptive strike ( or preemption doctrine ) .