During the Seven Years' War, Great Britain established a controversial legal principle known as the Rule of 1756. The rule declared that a neutral nation could not engage in trade during wartime if that specific trade was prohibited to them during peacetime. France, possessing vast colonial holdings in the Caribbean, maintained a strict mercantilist monopoly during peacetime, forbidding foreign vessels from trading directly with French colonies.
However, when the Royal Navy blockaded French ports and severed trade routes, France opened its colonial trade to neutral merchants, primarily from the Dutch Republic and later the young United States. Great Britain retaliated by enforcing the Rule of 1756, asserting that neutral ships carrying French goods were aiding the enemy effort and were therefore subject to capture and confiscation as legal prize. British privateers and naval forces seized hundreds of neutral vessels under this doctrine.
The British justification rested on the idea that neutral shipping was effectively acting as an arm of the French merchant fleet, overriding standard neutral rights. The Rule of 1756 sparked intense diplomatic pushback across Europe and America, leading neutral powers to form the League of Armed Neutrality to protect their shipping. Despite widespread condemnation, Britain maintained the policy through the Napoleonic Wars.
The doctrine laid the groundwork for modern international legal debates concerning belligerent rights, economic blockades, and the freedom of the seas during times of global conflict.