The Diana was a case heard by the United States Supreme Court in 1818. The dispute centered around a ship called the Diana, which had been seized by French privateers during the Napoleonic Wars. The owners of the vessel brought suit against France for damages caused to their property and sought compensation from them. In its decision, the court held that while it could not award damages directly to individuals who suffered losses due to foreign aggression, it did have authority under international law to grant reparations on behalf of those affected parties if they were unable or unwilling to bring suit themselves. This ruling established an important precedent for future cases involving claims against foreign governments and set forth principles regarding how such disputes should be handled in U.S courts going forward.

In The Diana, the Supreme Court was asked to decide whether a vessel that had been seized by French privateers during the Napoleonic Wars could be considered an enemy vessel. Chief Justice Marshall wrote for the majority and held that it could not because France had declared itself neutral in 1802 and therefore any vessels captured after this date were not subject to seizure as enemy vessels. However, Justice Johnson dissented from this opinion on two grounds: firstly, he argued that even if France had declared neutrality in 1802, its subsequent actions showed hostility towards other nations; secondly, he contended that since there was no treaty of peace between France and America at the time of capture (1806), all captures made prior to such a treaty should be treated as valid captures of enemy property. He concluded by stating his belief that Congress alone has authority over questions concerning war or peace with foreign powers and thus should have been consulted before making such a decision.