The U.S. diplomats complained to the British about what they perceived to be an unjustiﬁed destruction of the Caroline. The dispute was eventually resolved through a series of diplomatic notes exchanged between U.S. Secretary of State Daniel Webster and British Minister to Washington Henry Stephen Fox, in 1841. In one of his letters, Secretary of State Webster established four criteria for justiﬁable preemptive use of force. In Webster′s words, justiﬁable preemptive attack had to be:(1) “overwhelming” in its necessity; (2) leaving “no choice of means”; (3) facing so imminent a threat that there is “no moment for deliberation”; and (4) proportional. While acknowledging the weight and relevance of these factors, Lord Ashburton, the new British envoy, argued in his reply to Webster that the British attack on the Caroline did, in fact, meet these criteria. Webster, while not admitting that the incident had been justiﬁed, accepted the incongruous British apology.
美国外交官纷纷对英国人无端毁坏卡罗琳号一事表示责难，并质问对方作何解释。直到1841年，在美国国务卿丹尼尔-韦伯斯特与英国驻美大使亨利-史蒂芬-福克斯交换系列外交通牒后，这场争端才告平息。在其中一份通牒中，韦伯斯特提出了先占正当性的四条准则。在韦伯斯特看来，先发制人须得在以下四种情况下发生才算正当：（1).势在必行（2). 别无选择 (3). 遭遇无暇斟酌的紧急威胁 （4）. 与以上相当的他种情况。虽然承认这些因素重要且实用，新任英国特使阿什伯顿男爵在回应韦伯斯特时争论道，英国人对卡罗琳号的攻击事实上非常符合这些准则。韦伯斯特虽仍不认可这一事件的正当性，也只好暂时接受英方有失原则的道歉。
Colonel MacNab and Commander Drew would have been fully justiﬁed in arresting or expelling the rebels on Navy Island—British-Canadian territory after all—and destroying the Caroline if they had found it there. But the most comprehensive modern history of the events surrounding the Caroline incident correctly concludes that the British-Canadian attack “hardly met the requirements of self-defense set forth in Webster′s note of April 24, 1841.” We can readily see why. First, the British attack on the Caroline was by no means necessary. The British had signiﬁcantly superior forces; MacNab′s forces in Chippewa greatly outnumbered Mackenzie′s on Navy Island, with MacNab′s forces at 2,500 while estimates of Mackenzie′s forces ranged from 150 to 800.Second, the U.S. government did not intend to pose a threat to Canada. Instead, President Van Buren had instructed district attorneys to prosecute violators of U.S. neutrality laws and customs ofﬁcers to restrict incursions. While these efforts were not effective, there was nonetheless goodwill in Washington to improve relations with Great Britain. Third, there was no imminent threat to British-Canadian forces. Government forces earlier in the same month had routed the rebels at Montgomery′s Tavern near Toronto. When Drew failed to ﬁnd the Caroline at Navy Island, he could have returned for further orders—as some of his party did. Moreover, the provision of supplies to the Patriots by the Caroline did not pose an immediate threat, and there was no other evidence of an impending attack by the Patriots.
Thus even the Caroline incident itself did not meet the standards for which the case has become famous. The Caroline rules were instead constructed to assert American sovereignty. Despite the gap between the facts and the rules, Webster accepted the British statement (and regrets) in order to restore amity in U.S.-British relations so that more important issues—the northern frontier boundary, for instance—could be addressed. Nevertheless, the criteria delineated in Webster′s letter have become the gold standard for justiﬁable preemption in international law. Indeed, these criteria were applied by the Nuremberg Tribunal to deny Hitler′s claim to justiﬁable preemption in attacking Norway in 1940 and by the Tokyo Tribunal to justify the Netherlands′ preemptive declaration of war against Japan in 1941.The Caroline standards for imminence were also invoked when the Security Council condemned Israel′s preemptive strike against the Iraqi Osirak nuclear reactor in 1981.While there are good reasons for all three of these judgments, it is not clear that either passing or failing the Caroline test is one of them.
The Caroline standard is too extreme. It is not clear that an attack on U.S. territory was necessary as a matter of imminent self-defense. It is clear that the actual attack was not justiﬁed by the principles Webster and Ash-burton promulgated. Moreover, the principles themselves are deeply ﬂawed. They justify reﬂex defensive reactions to imminent threats and nothing more. For instance, they do not leave enough time for states to protect their legitimate interests in self-defense when they still do have some “choice of means,” albeit no peaceful ones, and some “time to deliberate” among the dangerous choices left. Extreme Caroline conditions are rarely found in reality. With the possible exception of the Netherlands′ declaration of war on Japan on December 8, 1941, accepted by the Tokyo Tribunal, I have not found one example of Caroline rules clearly validating an act of preemption. It is not irrelevant that the classic model for justiﬁable preemption was nothing of the sort. Indeed, the Caroline case better illustrates the difﬁculty of drawing a clear line separating imminent preemption from disallowed prevention.
The deeper problem with the Caroline strict standard of imminence is that it is standing in for the more ethically signiﬁcant considerations of danger and probability. This is the concern that Michael Walzer raises in his inﬂuential treatment of the 1967 Six-Day War between Israel and Egypt. Walzer and other scholars have made persuasive arguments that anticipatory uses of force by states are legitimate acts of national self-defense whenever a “failure to do so would seriously risk their territorial integrity or political independence.” This more expansive vision of preemption is justiﬁed when, Walzer adds, a “sufﬁcient threat” has been demonstrated, involving a manifest intent to injure; active preparation to attack on the part of the opponent; and a situation in which waiting signiﬁcantly magniﬁes the risk of great harm. These standards are a considerable step forward in matching the virtues of the rule of law with the security of states.
对于紧急准则苛刻的条件, 更严重的问题在于它正在日渐代替道德上对于危险与可能性更富意义的考虑，这也是迈克尔-华尔兹在那份针对1967年以色列-埃及六日战争颇具影响的对策中提出的担忧。华尔兹及其他学者提出了令人信服的论据，每当不预先使用武力领土完整或政治自治就受到威胁时，预先使用武力就成了国防的正当手段。华尔兹还指出，当一个所谓的”足够大的威胁”（比如，明显的破坏意图, 被对手视为积极备战，以及等待将大大增加重大伤亡的风险时）被宣示，即使更加扩大化的先发制人事件也会被证明为正当。这套准则极大地促进了法的精神与国家安全的协调。