The most significant use of German submarines during World War I was the campaign of unrestricted submarine warfare against Allied merchant shipping, particularly in 1917. The goal was straightforward: sink enough supply ships to starve Britain into surrender before American forces could tip the balance. At its peak, German U-boats were destroying over 600,000 tons of shipping per month, threatening to cut off the food and materials that kept Britain in the war.
The Blockade Strategy Behind the U-Boat Campaign
Britain imported the vast majority of its food and raw materials by sea. German military planners recognized this as a critical vulnerability. If U-boats could sink merchant vessels faster than the Allies could replace them, Britain’s ability to feed its population and supply its military would collapse. The strategy was essentially a counter-blockade: Britain had been using its surface navy to choke off supplies to Germany since 1914, and submarines gave Germany a way to do the same thing in reverse.
Germany first declared the waters around the British Isles a war zone in February 1915, warning that any ship found there could be sunk. But international backlash, especially from the United States, forced Germany to scale back. The policy shifted back and forth over the next two years as German leaders weighed the military benefits against the diplomatic costs. By January 1917, with the war grinding toward stalemate, the German high command forced a return to unrestricted attacks. The calculation was blunt: even if the campaign drew America into the war, Britain would be knocked out before American troops could arrive in meaningful numbers.
The Toll on Allied Shipping
The results in early 1917 were devastating. German U-boats aimed to sink more than 600,000 tons of shipping every month, and they came close. In June 1917, Allied losses hit 696,725 tons. Ships carrying grain, meat, fuel, and ammunition were going down faster than shipyards could build replacements. British food imports, which had stood at roughly 22 million tons per year before the war, were falling sharply. The threat was existential: if the trend continued, Britain faced genuine food shortages within months.
Germany built and deployed U-boats at an aggressive pace throughout the war, though the campaign came at a cost. A total of 178 U-boats were lost during the conflict, destroyed by mines, depth charges, ramming, and other countermeasures. But for much of 1917, new boats were entering service fast enough to keep pressure on Allied supply lines.
The Lusitania and Civilian Casualties
The most infamous single attack came on May 7, 1915, when a German submarine torpedoed the British passenger liner RMS Lusitania off the coast of Ireland. Of the 1,959 men, women, and children on board, 1,195 died, including 123 Americans. The sinking shocked the world and generated intense anger in the United States, though it did not immediately bring America into the war. It did, however, create lasting public hostility toward Germany that made later escalation far easier to justify politically.
How U-Boats Pulled America Into the War
The submarine campaign’s most far-reaching consequence had nothing to do with tonnage numbers. It brought the United States into the conflict. When Germany resumed unrestricted submarine warfare on January 31, 1917, President Wilson broke off diplomatic relations in February but still hesitated. Then, between the announcement and April, German submarines sank 10 American merchant ships. Three went down in a single weekend in March, and that appears to have been the tipping point for Wilson and his cabinet.
The submarine attacks alone might not have been enough. They combined with other provocations: Germany’s offer in the Zimmermann Telegram to help Mexico reclaim territory from the United States, widespread outrage over German conduct in Belgium, and a growing sense that the war could reshape the international order. But the U-boat campaign was the catalyst. America declared war on April 6, 1917, with the first troops arriving in France by June. The addition of American manpower and industrial capacity proved decisive in ending the war the following year, which is exactly the outcome Germany’s own military planners had gambled they could prevent.
The Convoy System That Turned the Tide
For months, the Allies struggled to counter the U-boat threat. Individual merchant ships sailing alone were easy targets. The solution, resisted for a surprisingly long time by the British Admiralty, was the convoy system: grouping merchant vessels together and surrounding them with armed escort ships. Once convoys became standard practice in mid-1917, losses dropped rapidly. From the peak of nearly 697,000 tons in June, sinkings fell to about 556,000 tons in July, then 472,000 in August, 354,000 in September, and down to 303,000 tons by November, the lowest monthly total in over a year.
The convoy system did not eliminate the U-boat threat, but it broke the math that had made the campaign viable. Germany could no longer sink ships fast enough to outpace Allied shipbuilding and imports. The window for starving Britain into submission closed, and the arrival of American forces ensured it would not reopen.
Why the U-Boat Campaign Defined Submarine Warfare
Before World War I, submarines were an unproven technology with no clear strategic role. The German U-boat campaign demonstrated that a relatively small fleet of submarines could threaten a global naval power’s survival by targeting its supply lines rather than its warships. This was a new kind of warfare: not fleet battles between battleships, but an economic war of attrition waged beneath the surface against cargo ships and civilian vessels.
The lessons shaped naval strategy for decades. Britain entered World War II acutely aware of its vulnerability to submarine blockade, and Germany launched an even larger U-boat campaign in the Atlantic starting in 1939. Every major navy in the world invested heavily in submarine technology and anti-submarine defenses in the interwar years, all because of what a few hundred German submarines had nearly accomplished between 1914 and 1918.