Promulgated by Great Britain during the Seven Years' War, the Rule of 1756 asserted that a neutral nation could not engage in time of war in a trade from which it was excluded in time of peace. Because France strictly prohibited foreign nations from trading with its Caribbean colonies during peacetime, Britain declared that any neutral ships carrying French colonial goods during wartime were subject to capture. British prize courts ruled that neutral vessels attempting to bypass French commercial restrictions were acting as auxiliary merchant fleets for France.
This doctrine caused immense strain between Britain and neutral powers, eventually influencing maritime law disputes during the French Revolutionary Wars and the War of 1812. The rule effectively turned peacetime commercial monopolies into wartime blockades enforced by British naval hegemony, setting precedents for belligerent rights at sea.