Caroline test

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 test takes its name from the Caroline affair.

In 1837, settlers in Upper Canada rebelled against the British colonial government. 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 British force from Canada entered United States territory at night, seized the Caroline, set the ship on fire, and sent it over Niagara Falls. At least one American was killed. The British claimed that the attack was an act of self-defense. In a letter to the British Ambassador, Secretary of State Daniel Webster argued that a self-defense claimant would have to show that the:

necessity of self-defense was instant, overwhelming, leaving no choice of means, and no moment of deliberation ..., and that the British force, even supposing the necessity of the moment authorized them to enter the territories of the United States at all, did nothing unreasonable or excessive; since the act, justified by the necessity of self-defense, must be limited by that necessity, and kept clearly within it.

The terms "anticipatory self-defense", "preemptive self-defense" and "preemption" traditionally refers to a state's right to strike first in self-defense when faced with imminent attack. 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". This has later come to be referred to as "instant and overwhelming necessity".

The principle of self-defense had been acknowledged prior to the Caroline test, but it was notable for setting out specific criteria by which it could be determined whether there had been a legitimate exercise of that right. The test was accepted by the United Kingdom and came to be accepted as part of customary international law.

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