Unrestricted Submarine Warfare (1917): Germany's U-Boat Gamble

Unrestricted submarine warfare was the German policy, begun on 1 February 1917, of sinking without warning any merchant ship, Allied or neutral, in declared zones around the British Isles, France and the Mediterranean. Germany hoped to starve Britain out of the First World War within months; instead, the campaign was contained by the convoy system and helped bring the United States into the war.

Background

Under the traditional “prize rules”, a warship had to stop a merchant vessel, check its cargo and ensure the safety of its crew before sinking it. Submarines, small and vulnerable on the surface, found these rules hard to follow. In February 1915 Germany declared the waters around Britain a war zone where ships could be sunk without warning.

The sinking of the Lusitania on 7 May 1915, with 128 Americans among nearly 1,200 dead, caused fury in the United States. After further incidents, including the torpedoing of the French ferry Sussex in March 1916, Germany promised in the so-called Sussex pledge to respect the prize rules. The British blockade, meanwhile, was causing serious shortages in Germany.

The decision

By late 1916 the war on land was deadlocked after Verdun and the Somme, and the German surface fleet had failed to break British control of the North Sea at Jutland. The chief of the Admiralty Staff, Admiral Henning von Holtzendorff, argued in a memorandum of December 1916 that sinking about 600,000 tons a month would force Britain to make peace within five months, before American help could matter.

Field Marshal Paul von Hindenburg and General Erich Ludendorff supported the plan. Chancellor Theobald von Bethmann Hollweg doubted it but gave way. At a conference at Pless on 9 January 1917, Kaiser Wilhelm II approved unrestricted submarine warfare, and it was announced to neutral governments on 31 January.

The campaign at its height

Germany started with just over 100 operational U-boats, and sinkings rose sharply. In April 1917 alone, U-boats sank around 860,000 gross tons of Allied and neutral shipping, the worst month of the war. Admiral Sir John Jellicoe, the First Sea Lord, warned that Britain could not go on if losses continued.

The United States broke off relations with Germany on 3 February 1917. After American ships were sunk and the Zimmermann Telegram was published, Congress declared war on 6 April 1917.

How the convoy system worked

The Royal Navy had long resisted convoys, believing that large groups of ships would be easy targets and that there were too few escorts. Analysis in early 1917 showed that the number of ocean voyages needing protection was smaller than feared. An experimental convoy sailed from Gibraltar in May 1917, and regular convoys were introduced over the summer.

Convoys worked because the ocean is vast: a group of ships was barely easier to spot than a single one, so many U-boats found nothing. When they did attack, escorting destroyers were waiting with depth charges. American destroyers, sent to Queenstown (now Cobh) in Ireland from May 1917 under Admiral William Sims, added badly needed escorts.

Losses fell steadily from the second half of 1917. Mines, hydrophones, air patrols and the Dover Barrage also took a growing toll of U-boats.

Life on the home front

The campaign had a direct impact on daily life in Britain. Food imports fell, prices rose and queues formed outside shops. The government expanded home food production, ploughed up grassland and encouraged allotments. Rationing of sugar began at the start of 1918, and meat and butter were rationed in parts of the country soon after, before a national scheme was introduced.

Consequences

The gamble failed. Britain was never starved into surrender, and shipbuilding in Britain and the United States eventually outpaced losses. Germany lost about 178 U-boats during the war, with some 5,000 crew killed, while American troops crossed the Atlantic in convoys with very few losses.

The campaign’s political cost was decisive. It turned the United States, the world’s largest economy, into a belligerent. Germany adopted the same strategy again in the Second World War, when convoys once more proved the key to defeating it.

Timeline

• 1915, 4 February: Germany declares the waters around Britain a war zone.
• 1915, 7 May: A U-boat sinks the Lusitania.
• 1916, May: Germany gives the Sussex pledge.
• 1917, 9 January: The Kaiser approves unrestricted submarine warfare at Pless.
• 1917, 1 February: The campaign begins.
• 1917, 3 February: The United States breaks off relations with Germany.
• 1917, 6 April: The United States declares war on Germany.
• 1917, April: Shipping losses reach their wartime peak of around 860,000 tons in a month.
• 1917, May: The first experimental convoy sails from Gibraltar.

Why it matters

Unrestricted submarine warfare was a turning point of the First World War. It nearly cut Britain’s lifeline, but it brought America into the conflict and showed that the humble convoy could defeat the submarine.