The greater the threat, the greater is the risk of inaction -- and the more compelling the case for taking anticipatory action to defend ourselves, even if uncertainty remains as to the time and place of the enemy's attack. To forestall or prevent such hostile acts by our adversaries, the United States will, if necessary, act preemptively." (cited in Ryan, 2009)

Ryan relates that the rule of anticipatory self-defense was described in letters exchanged between Britain's Lord Ashurton and U.S. secretary of state Daniel Webster in 1837 in an indicated involving the private U.S. ship Caroline that was destroyed by British forces when it was found to be providing aid to Canadian rebels who were fighting against the Crown. The diplomatic correspondence set to the British Ambassador and which protested the attack it was written by Webster that the "...anticipatory self-defense may be invoked only where the threat is imminent,...
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