Customary rules were originally laid down in the days of sailing ships. These were supplemented by various international agreements including Declaration of Paris (1856) and the Hague Conventions (1899 and 1907) and other naval agreements during the 20th century. Although these rules are still part of the laws of war, during the two world wars, changes in technology: radio and the submarine made the rules redundant between belligerents although the Nuremberg Tribunal found that they were still applicable to neutral merchant shipping.
Prize rules include state that: passenger ships may not be sunk; crews of merchant ships must be placed in safety before their ships may be sunk (life boats are not considered a place of safety unless close to land); only warships and merchant ships that are a threat to the attacker may be sunk without warning.
In 1912, British Admiral Sir John "Jackie" Fisher, by then a retired First Sea Lord, presented a paper to the Cabinet. He developed the argument that submarines would find adherence to Prize Rules impossible, for practical reasons: a submarine could not capture a merchant ship, for it would have no spare manpower to deliver the prize to a neutral port, neither could it take survivors or prisoners, for lack of space. "...there is nothing a submarine can do except sink her capture." If a merchant ship were armed, as was permitted by a conference in London in 1912, then a submarine was under more pressure to destroy a ship. He asked: "What if the Germans were to use submarines against commerce without restriction?"
This last comment was thought to be unsupportable. Winston Churchill, then First Lord of the Admiralty and political head of the Royal Navy, supported by senior naval opinion, said it was inconceivable that "...this would ever be done by a civilised power." It was Fisher who was proven correct.
The treaties are still in effect today.