The Buena Ventura case in 1899 revolved around the seizure of a Spanish vessel, The Buena Ventura, by U.S. naval forces during the Spanish-American War before an official declaration of war had been made. The owners of the ship argued that because no formal state of war existed at the time their property was seized, it should be returned to them. However, the Supreme Court ruled against this argument stating that while Congress had not formally declared war when The Buena Ventura was captured, hostilities were already underway and thus a de facto state of war existed. Therefore, under international law principles regarding maritime warfare and prize cases (cases involving capture at sea), such seizures were legal even without a formal declaration from Congress. This ruling established precedent for future conflicts where military action might precede an official declaration.

In the dissenting opinion for The Buena Ventura case, Justice Harlan argued that the court had overstepped its jurisdiction by ruling on a matter of international law. He contended that it was not within the purview of U.S courts to decide whether or not Spain's blockade during the Spanish-American War was effective and thus legal under international law. Instead, he believed this decision should have been left to political branches of government - specifically, Congress and President who are constitutionally authorized to declare war and make peace. Furthermore, he expressed concern about potential diplomatic implications if U.S courts were seen as interfering in matters traditionally reserved for executive branch discretion.