At the onset of the American Civil War, President Abraham Lincoln faced a legal paradox: declaring a blockade under international law traditionally implied a formal state of war between sovereign nations, which might grant the Confederacy international recognition. However, without a blockade, the Union could not legally seize foreign ships trading with Southern ports. In 1863, the Supreme Court addressed this dilemma in the Prize Cases, establishing groundbreaking precedent for executive power and maritime sovereignty.
The Court examined four merchant vessels seized by Union warships while attempting to breach the naval blockade. The central issue was whether the President possessed the executive authority to institute a blockade and declare enemy property subject to capture on the high seas without a formal declaration of war by Congress. In a 5-4 decision, Justice Robert Cooper Grier ruled that a state of civil war is a factual condition rather than a formal legal status.
The President was obligated to respond to actual hostilities regardless of official terminology. Under maritime prize law, enemy status was determined by belligerent residency rather than individual allegiance. This created a dual status where Southern residents were legally rebels domestically, yet foreign neutral vessels trading with them could be lawfully seized as hostile prizes.