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U-boats and unrestricted submarine warfare
1915; resumed February 1917· Idea, religion or movement· Strategy & tactics
Well documentedSolid records, physical evidence or scientific consensus.
Germany used submarines (U-boats, from Unterseeboot) to sink merchant ships without warning, trying to starve Britain into surrender.
See it on the Battles & Wars map →What happened
Old rules of war said a raider must stop a merchant ship and let the crew escape before sinking it. Submarines, fragile on the surface, could not safely do this. In February 1915 Germany declared the waters around Britain a war zone. The sinking of the liner Lusitania that May, killing 1,198 people including 128 Americans, outraged the US, and Germany pulled back. In February 1917 it resumed unrestricted attacks. Convoys, merchant ships sailing together under escort, defeated it.
Why it matters
It brought the United States into the war in April 1917 and became the model for far larger submarine campaigns against shipping in World War II.
How we know
German naval records, British Admiralty shipping-loss statistics, the wreck of the Lusitania off southern Ireland, and US diplomatic notes protesting the attacks.
Still debated
The Lusitania carried rifle ammunition and shell parts; whether they caused the mysterious second explosion that sank it so fast is still debated.
Connections (8)
▸Battle of Jutland pushed Germany toward U-boats and unrestricted submarine warfareMostly agreed
The connection
After Jutland, Admiral Scheer reported to the Kaiser that even a successful fleet battle would not force Britain to make peace; only a U-boat war on British trade could. Over the next months Germany's leaders came round to this view, and in February 1917 they resumed unrestricted submarine warfare.
When and how
Scheer's report of July 4, 1916, and the German leadership's decision at Pless in January 1917, after Jutland in the North Sea.
How sure are we?
Scheer's report survives and is quoted in his memoirs; the January 1917 decision is recorded in German government minutes.
Debates and open questions
Other pressures mattered too: food shortages, the army's demands and the failure at Verdun. Jutland was one push among several.
▸U-boats and unrestricted submarine warfare brought the United States into World War IWell documented
The connection
After Germany resumed unrestricted attacks in February 1917, U-boats began sinking American ships. Together with the Zimmermann Telegram, in which Germany offered Mexico an alliance against the US, this turned American opinion. Congress declared war on April 6, 1917, and American troops, money and supplies helped tip the balance in 1918.
When and how
Atlantic shipping lanes and Washington, February to April 1917; President Woodrow Wilson asked Congress for war on April 2.
How we know
Wilson's war message to Congress names submarine warfare as the chief cause; German records show leaders expected US entry and accepted the risk.
Debates and open questions
Whether America would have joined anyway, given its loans and trade with Britain, is debated.
▸U-boats and unrestricted submarine warfare returned on a vast scale in World War IIWell documented
The connection
Germany again tried to starve Britain with submarines. Admiral Karl Dönitz, a U-boat veteran of 1918, used 'wolf packs' that attacked convoys together at night. In the Battle of the Atlantic, U-boats sank roughly 2,800 Allied merchant ships, but radar, code-breaking, air patrols and escorts defeated them by mid-1943. The US waged the same kind of war against Japan.
When and how
The Atlantic, 1939–1945; the turning point came in 'Black May' 1943, when Allied escorts and aircraft sank over 40 U-boats.
How we know
German and Allied naval records, decrypted Enigma messages and loss tables; roughly 30,000 of about 40,000 U-boat crewmen died, a well-documented toll.
Debates and open questions
Historians debate how close the U-boats came to winning, and how much credit belongs to code-breaking versus radar, aircraft and mass shipbuilding.
▸Battle of the Atlantic was fought against U-boats and unrestricted submarine warfareWell documented
The connection
Germany revived the unrestricted submarine warfare of World War I, sinking merchant ships without warning. Admiral Karl Dönitz grouped U-boats into wolfpacks that shadowed convoys and attacked together at night. The Allies answered with the remedy that had worked in 1917, convoys, now backed by radar, aircraft and codebreaking.
When and how
North Atlantic, Arctic and American coastal waters, 1939–1945; Dönitz's U-boat command against British, Canadian and US navies and air forces.
How we know
German U-boat command war diaries, Allied convoy records, and the Nuremberg trial, where Dönitz's submarine orders were examined.
Debates and open questions
Some argue that if Germany had built many more U-boats before 1939, as Dönitz wanted, Britain could have been starved out.
▸Submarine made possible U-boats and unrestricted submarine warfareWell documented
The connection
By 1914 diesel-electric submarines with self-propelled torpedoes could cruise far out to sea. On September 22, 1914, Germany's U-9 sank three British cruisers in about an hour, killing some 1,450 men. Germany then turned U-boats against merchant shipping, sinking roughly 5,000 ships during the war and nearly starving Britain in 1917.
When and how
North Sea and Atlantic, 1914–18; Germany's U-boat fleet against British, Allied and neutral shipping.
How we know
Admiralty and German naval records, ship-loss lists compiled after the war, and wrecks on the seabed.
Debates and open questions
Whether a U-boat campaign launched earlier and with more boats could have won the war for Germany is debated.
▸Zimmermann Telegram (broken by Room 40) accompanied Germany's return to U-boats and unrestricted submarine warfareWell documented
The connection
The telegram's first line announced Germany's plan: from February 1, 1917, U-boats would sink any ship, neutral or not, in the waters around Britain. Germany's leaders expected this might bring in the United States, so they offered Mexico an alliance to keep Americans busy at home. They gambled that U-boats would starve Britain before American troops arrived.
When and how
Pless Castle, Silesia, January 9, 1917, where Kaiser Wilhelm II approved the campaign; Zimmermann's telegram, January 16, 1917.
How we know
The telegram's text and German records of the Pless decision, including the navy's memo predicting Britain's collapse within months.
Debates and open questions
Whether German leaders really expected Mexico or Japan to act is doubtful; Mexico's president, Venustiano Carranza, studied the idea and declined.
▸Sonar (ASDIC) was developed to hunt the U-boats and unrestricted submarine warfareWell documented
The connection
U-boats sank thousands of Allied ships in World War I. Listening hydrophones could hear a submarine but not pinpoint it, so France and Britain raced to develop echo-ranging, sending sound pulses and timing their echoes. It came too late for that war, where convoys beat the U-boats, but it was standard on escorts by 1939.
When and how
France (Paul Langevin and Constantin Chilowsky) and Britain's Admiralty Board of Invention and Research, 1915–18.
How we know
Patents, Admiralty research records, and test reports describing the first echoes from submarines in 1918.
Debates and open questions
Credit is shared and disputed among Langevin, the British team, and Canadian-born Reginald Fessenden, whose 1914 sound oscillator detected an iceberg.
▸Krupp of Essen (Alfred Krupp, the 'Cannon King') built U-boats at its Kiel yard for U-boats and unrestricted submarine warfareWell documented
The connection
Krupp's Germaniawerft shipyard in Kiel, fully owned by the firm from 1902, built the German navy's first U-boat, U-1, commissioned in 1906. During World War I it was one of the main U-boat builders, turning out dozens of boats for the campaigns against Allied and neutral shipping.
When and how
Kiel, on Germany's Baltic coast, 1906–18; Krupp's Germaniawerft yard, building for the Imperial German Navy.
How we know
Shipyard records and German naval lists record each boat's builder; U-1 itself survives in the Deutsches Museum in Munich.
Debates and open questions
Historians debate how far Krupp and other firms pushed for naval expansion for profit, rather than following the navy's lead.