The Caroline test is a 19th-century formulation of customary international law, reaffirmed by the Nuremberg Tribunal after World War II, which said that the necessity for preemptive self-defense must be "instant, overwhelming, and leaving no choice of means, and no moment for deliberation."
The United States remained officially neutral about the rebellion, but American sympathizers assisted the rebels with men and supplies, transported by a steamboat named the Caroline.
In response, a combined Anglo-Canadian force from Canada entered United States territory at night, seized the Caroline, set the ship on fire, and sent it over Niagara Falls.
[3] In order to justify such an action, the Caroline test has two distinct requirements: In Webster's original formulation, the necessity criterion is described as "instant, overwhelming, leaving no choice of means, and no moment of deliberation".
[10] In 2008, Thomas Nichols wrote: Thus the destruction of an insignificant ship in what one scholar has called a 'comic opera affair' in the early 19th century nonetheless led to the establishment of a principle of international life that would govern, at least in theory, the use of force for over 250 years [sic].