The Amiable Nancy was a landmark Supreme Court case that established the principle of maritime law in the United States. The case concerned a ship called the Amiable Nancy, which had been seized by French privateers during an armed conflict between France and Great Britain. The owners of the vessel sued for damages in U.S. courts, claiming that their rights as neutral parties were violated when their property was taken without compensation or due process of law. In its decision, the court held that under international maritime law, vessels belonging to neutral nations could not be seized by belligerents unless they were engaged in hostile activities against them or otherwise posed a threat to their interests at sea. This ruling set an important precedent for future cases involving claims related to international conflicts and helped establish American neutrality on matters concerning foreign affairs and trade disputes with other countries.

In The Amiable Nancy, the Supreme Court was asked to decide whether a ship's master had the authority to sell cargo without permission from its owner. In a 5-4 decision, Justice Story wrote for the majority that such sales were valid and binding on all parties involved. However, in his dissenting opinion Chief Justice Marshall argued that while it may be true that masters of ships have some degree of autonomy when it comes to selling goods at sea, they do not possess absolute power over their vessels or cargoes. He maintained that any sale made by a master must still be approved by either an agent or principal before being considered legally binding; otherwise there is no guarantee as to who will bear responsibility for any losses incurred due to such transactions. Furthermore, he noted how this case could set precedent which would allow unscrupulous captains and owners alike to take advantage of unsuspecting merchants with impunity if these sales are allowed without proper authorization from those responsible for them.