The Mary Ann. Plumer was a case heard before the United States Supreme Court in 1823. The claimant, Mary Ann Plumer, sought damages for losses she suffered when her ship and cargo were seized by the British during the War of 1812. She argued that since Britain had violated international law by seizing her property without due process or compensation, she should be compensated for her losses from either Britain or the United States government. The court ultimately ruled against Ms. Plumer on two grounds: firstly, that it lacked jurisdiction to hear such cases; and secondly, that even if it did have jurisdiction to hear them they would not grant relief because of sovereign immunity principles which protect governments from being sued in their own courts unless they consent to do so explicitly beforehand.

In The Mary Ann. Plumer, Claimant case, the Supreme Court was asked to decide whether a vessel and cargo seized by French privateers during the Quasi-War with France should be restored to its original owners. Justice Johnson delivered a dissenting opinion in which he argued that since there had been no formal declaration of war between the United States and France at the time of seizure, it could not be considered an act of hostility against either nation. He further argued that even if such hostilities did exist, they were limited only to vessels sailing under their respective flags; therefore any neutral vessel captured would have been entitled to restitution regardless of who owned it or what flag it sailed under. Furthermore, he stated that since neither party had declared war on each other prior to this incident, both parties must abide by international law which requires restitution for all goods taken without just cause from innocent third parties like Mrs Plumer's ship and cargo in this case.