Heres what imminent threat looks like legally

Judgement about the legality of armed force in instances of self-defense typically have to pass what is often referred to as “the Caroline test” of imminence. In 1837, British forces attacked a U.S.-flagged steamboat (the SS Caroline) being used to supply rebels in Upper Canada against the British colonial government. In his famous analysis of the incident, the U.S. Secretary of State Daniel Webster exculpated the British. “[E]ven supposing the necessity of the moment authorized them to enter the territories of the United States at all, [they] did nothing unreasonable or excessive.” The act was justified, inasmuch as the “necessity of self-defense was instant, overwhelming, leaving no choice of means, and no moment of deliberation.”  Subsequent international legal development has generally embraced this idea insofar as self-defense is allowed in anticipation of attacks that are imminent, though the precise contours of this standard remain contested.

Much more problematic is the launching of a “preemptive” attack against a threat that is developing but not yet imminent. A decade ago, in its 2002 National Security Strategy of the United States [PDF], the administration of George W.Bush enunciated a right to “preemption”. The basis of this controversial doctrine was that in an age of catastrophic threats, the United States needed the leeway to launch armed attacks to protect itself from catastrophic threats that were emerging but not yet fully realized.