Rules of War

The rules governing naval warfare also leave much discretion to the participants. Although belligerent warships may attack and sink an enemy warship encountered on the high seas, they may neither attack nor sink an enemy merchant ship unless it refuses to obey a signal to stop and submit to inspection. Conversely, belligerent merchant ships are not obliged to stop or submit to inspection but may attempt to escape or act in self-defense. However, the line separating an act of self-defense from an offensive maneuver is subject to some debate. In 1916 a British merchant ship captain was court-martialed for ramming a German U-boat, despite the captain's claim that his vessel was acting in self-defense.

When an enemy warship has been captured, it becomes the property of the captor and may be sunk or brought into port. If an enemy merchant ship is captured, it must be taken into port for adjudication regarding the ownership of the vessel and its cargo pursuant to international law. In either case the passengers and crew of a captured ship may not be harmed. Captured members of enemy naval forces are entitled to treatment as prisoners of war. Shipwrecked belligerents are also entitled to humane treatment under the rules of war and may not be abandoned or refused quarter. Many of the same rules governing surface warships have been applied to submarine warfare as well.