In 1756, facing conflict with France during the Seven Years' War, Great Britain enacted a groundbreaking legal doctrine known as the Rule of 1756. Before the war, France restricted trade with its valuable Caribbean colonies, like Saint-Domingue, strictly to French vessels under mercantilist doctrine. However, when British naval blockades crippled French shipping, France attempted to bypass the blockade by opening its colonial ports to neutral Dutch traders.
Britain responded with the Rule of 1756, asserting that a neutral nation could not engage in wartime trade with a belligerent's colonies if that trade was prohibited during peacetime. Britain argued that neutral ships aiding France effectively became enemy transports, making their cargoes subject to seizure by the Royal Navy. Neutral nations, particularly the Dutch, protested furiously against what they viewed as a heavy-handed violation of free trade on the high seas.
Despite international outcry, the doctrine endured and was repeatedly invoked during the Napoleonic Wars. It heavily influenced the concept of neutral rights and contributed directly to maritime disputes between Great Britain and the United States, ultimately setting the stage for the War of 1812. The Rule of 1756 remains a crucial milestone in the evolution of prize law, contraband, and international maritime neutrality.