The Legal Lock



Self-defense | How does law protect in war? Caroline Test

“I know not with what weapons world war 3 will be fought, but world war 4 will be fought with sticks and stones”  – EINSTEIN


This study will explore the famous case called the Caroline Affair (The Caroline Case). The Caroline case is dominant in international customary law because the doctrine arises out of the case called the doctrine of self-defence. This article will attempt to study the Caroline case and will establish elements like necessity or proportionality as an unarguably important aspect to use force with respect to self-defence.

In common parlance, self-defence in customary parlance can be understood as the inherent power or right of the country to use the means of force in response to an armed attack from another state.


The timeline of the event starts from December 29, 1837, involving the state of the United States of America, the state of the United Kingdom and the Canadian rebels from the Canadian independence movement. In the year 1837, when some Canadian rebels including William Lyon Mackenzie (he was a Scottish-born Canadian-American journalist and politician) fled to the Niagara River, the river is the part of the border between the province of Ontario in Canada (on the west) and the state of New York in the United States (on the east), in the ship Caroline, a small steamboat.

These rebels were supported by the natives of America. To supply the men at Navy Island, the rebels hired Caroline, a small steamboat, to bring support and more troops. Further, British militia from Upper Canada crossed to the US shore of the Niagara River and set adrift a small rebel-operated vessel, the Caroline (which drifted over the Niagara Falls).

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A brief struggle ensued where several Americans were injured and an American named Amos Durfee was killed. In retaliation, a private militia composed of both US citizens and Canadians attacked a British vessel and destroyed it. In 1838, there were several other clashes pitting British forces against the private militia. Historians recall after the death of Amos Durfee, Americans were very infuriated.

The question among Americans was who murder Amos Durfee. They were considering this death a murder instead of an act of war. The diplomatic relations between the state of the United States of America and the state of the United Kingdom ran high for many years.

A Scottish-Canadian who served as sheriff in Niagara, Ontario, Alexander McLeod bragged about taking part in the attack against Amos Durfee. Further, Alexander McLeod was arrested by American authorities for the murder and arson. Under these conditions, it was doubtful that McLeod would get a fair trial and Britain felt that the release of McLeod was “indispensable to British honour”. 

The Caroline issue further was only worsening during the Martin Van Buren (an eighth president of the United States) administration. The British Government demanded his repatriation leading Lord Palmerston (he served as the Prime minister of the United Kingdom two times) to threaten that a continued refusal to repatriate McLeod would result in “war immediate and frightful in its character because it would be a war of retaliation and vengeance”.

At several points over the years of the dispute, the war between the United States and Great Britain became a live possibility. Diplomatic efforts, taken over in 1841 by U.S.Secretary of State Daniel Webster and a new British envoy to the United States, Lord Ashburton (Alexander Baring), produced an agreement on the law, and agree to disagree on the facts. Soon thereafter the border issues were largely resolved by the Webster-Ashburton Treaty. 


In the aftermath of the attack, the negotiation started between the diplomat of the state of the United State of America, Secretary of State John Forsyth and the British diplomat to the United States of America Henry Stevan Fox.

But it took the great intellectual, diplomatic and legal firepower of their much more esteemed replacement- combined with the timing of their respective government’s revitalized desire to resolve transatlantic friction- to help provide some authoritative clarity regarding the customary law of the resort to military force.

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The big issue Caroline issued was the arrest of McLeod. The replacements Lord Ashburton who was British secretary at that time and Daniel Webster, American Secretary of State had several negotiations on international forums.

The British asserted that the New York state courts could not try McLeod and that he should be sent back to Canada because the British readily took full responsibility for the attack. Although the United States federal government agreed, the New York authorities were not willing to cooperate.  


The doctrine which arises from this case is commonly known as the “The Caroline Doctrine”. Daniel Webster in his letter to Fox wrote: 

“Under these circumstances, and under those immediately connected with the transaction itself, it will be for her majesty’s government to show upon what state of facts and what rules of national law, the destruction of the “Caroline” is to be defended.

It will be for that government to show a necessity of self-defence, instant, overwhelming, leaving no choice of means, and no moment for deliberation. It will be for it to show, also, that the local authorities of Canada, 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-defence, must be limited by that necessity, and kept clearly within it.

It must be shown that admonition or remonstrance to the persons on board the “Caroline” was impracticable, or would have been unavailing.

It must be shown that daylight could not be waited for; that there could be no attempt at discrimination between the innocent and the guilty; that it would not have been enough to seize and detain the vessel; but that there was a necessity, present and inevitable, for attacking her in the darkness of the night, while moored to the shore, and while unarmed men were asleep on board, killing some and wounding others, and then drawing her into the current, above the cataract, setting her on fire, and careless to know whether there might not be in her the innocent with the guilty, or the living with the dead, committing her to a fate which fills the imagination with horror.

A necessity for all this the government of the United States cannot believe to have existed.”

The test was accepted by the United Kingdom and came to be accepted as part of customary international law. The Caroline test has two distinct requirements:

  • 1. The use of force must be necessary because the threat is imminent and thus pursuing peaceful alternatives is not an option (necessity)
  • 2. The response must be proportionate to the threat (proportionality)


Many historians and scholars believed that Daniel Webster and Lord Ashburton agreed on this principle merely to avoid war between states which could result in a great loss for both countries. The takeaway for Britain and foreign sovereigns was, in effect, that America’s federal government was not master of its own house in the conduct of its foreign affairs.

Today, the Caroline affair is considered the customary international legal principle for anticipatory self-defence. Under Article 51 of the UN Charter, states have the right to self-defence “if an armed attack occurs” the choice of words used means that an attack has not yet started, and the use of force on a sovereign state without being attacked does not qualify under international law (with the exception of a UN Security Council resolution). However, this conclusion is not universally accepted.


The Caroline doctrine was not even a correct recitation of international law at the time of the Caroline incident and must be viewed in the context of the political events taking place. Webster’s definition was divergent from the ideas of other international law scholars at the time.

While Henry Wheaton’s 1846 treatise on international law recognized an absolute right to self-defence as an extension of the absolute right and duty of a state for self-preservation, he never once mentioned the Caroline incident as an example of customary international law.

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