Unrestricted submarine warfare

Unrestricted submarine warfare is a naval strategy in which submarines attack enemy and neutral shipping without warning, rather than following prize rules requiring warning and provision for crews’ safety.

Prize rules: International rules governing the capture of merchant ships during wartime, including safeguards for passengers and crews. These rules required submarines to stop and search merchant ships before sinking them.
Naval warfare: Warfare conducted by or against forces operating at sea. Submarine attacks became a distinct form of naval strategy as submarines entered combat.
Zimmermann Telegram: A 1917 German diplomatic message proposing a military alliance with Mexico if the United States entered the war. Its revelation compounded American anger after Germany resumed unrestricted attacks.
Restricted submarine warfare: Submarine operations that observe warning and safety procedures for merchant vessels under the law of naval warfare. It preserves the procedures that unrestricted attacks deliberately abandon.
Submarine: A watercraft capable of operating underwater, commonly using submersion to evade detection or attack. Its limited survivability while surfaced made compliance with warning rules dangerous.
Submarine warfare in World War I: The use of submarines in combat during the First World War, including attacks on warships and merchant shipping. Early wartime experience shaped Germany’s later decision to remove warning restrictions.
United States entry into World War I: The United States’ declaration of war against Germany on April 6, 1917, and its military entry into the First World War. The renewed submarine campaign was a principal factor in the American decision for war.
Commerce raiding: Naval attacks on an enemy’s commercial shipping to disrupt trade and supply. Both strategies target maritime commerce, but commerce raiding does not inherently require attacks without warning.
Torpedo: A self-propelled underwater weapon designed to strike ships or other targets. Torpedoes let submarines attack without approaching merchant ships to board them.
Lusitania: A British passenger liner sunk by a German submarine off Ireland in May 1915, killing nearly 1,200 people. Its sinking intensified international outrage over attacks on passenger ships without warning.

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