Submarines in World War I: A Detailed Summary

Submarines in World War I were an important weapon and played a significant role in the history of the conflict, especially in the northern Atlantic Ocean.  In particular, Germany used submarines (U-BoatsGerman submarines whose name is shortened from ‘Unterseeboot,’ meaning undersea boat.) extensively throughout the naval battles of World War I. World War I was a deadly conflict as shown by the millions of military and civil casualties it caused.  The large number of casualties was the result, in part, of the development and use of weaponry that took place during the war.  In all, the submarine was one of the most significant weapons of World War I alongside other important weapons, such as: airplanes, airships, machine guns, poison gas, rifles, and tanks.​

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Smart Allied response that kept supplies moving★ merchant shipskey term!

Convoy

(noun)

A convoy is a group of merchant ships sailing together so that Royal Navy warships can protect them from enemy attack. Traveling in a group made it harder for submarines to sink supply ships one by one.

in a sentence: Britain grouped its supply ships into convoys to reduce losses from German U-Boat attacks in the North Atlantic.

Source: Imperial War Museums© factsthatcrunch.comMore World War I Facts →

German Submarines (U-Boats) in World War I

As stated above, Germany used submarines in World War I to wage naval war against its rivals: Britain and France.  In German, these submarines were called ‘Unterseeboot’, which is usually shortened to ‘U-Boats’.  In general, the German navy produced several classes of U-Boats.  For instance, some were produced with diesel engines, while others were powered by kerosene.  As well, some German U-Boats were classified as merchant types, while others were classified as attack types.  Finally, some were minelayersSubmarines that secretly placed explosive naval mines in strategic locations to block or destroy enemy ships. and placed naval mines in strategic locations to defend sites against Allied naval forces.  Regardless, German U-Boats wreaked havoc on the Allied naval and merchant shipsCivilian cargo vessels that carried supplies, raw materials, and goods across the ocean during wartime. in the North Atlantic Ocean.  For instance, an important factor of the German Navy during World War I was its U-Boat Campaign from 1914 until 1918.  In general, the U-Boat Campaign by Germany was an attack against the merchant shipping routes of the Allied nations.​

The clearest example of German submarine warfare during World War I and its impact was the sinking of the Lusitania in May of 1915.  The ship left New York City on May 1st, 1915 and was bound for Liverpool, England.  While the Lusitania was a passenger liner that operated in the North Atlantic it was also being used, as was common practice at the time, to ferry war time munitionsMilitary weapons and ammunition, secretly carried aboard the Lusitania alongside civilian passengers. and supplies from America to Britain.  Germany believed the ship was carrying munitions and supplies, and on May 7th, 1915, a German submarine (U-Boat U-20) fired on the Lusitania just off the coast of Ireland.  In all, 1,198 people aboard the ship died as it took just 18 minutes for the passenger liner to sink beneath the surface.  The dead included 128 Americans who had been aboard the ship at the time.​

As the war progressed, Germany attempted to push the war in its favor by limiting the supplies and raw materials that Britain and France received from their colonies.  As such, Germany used its U-Boats to attack and sink merchant ships as a means of hurting the war effort of these nations.  In fact, since Britain was an island nation it required a steady supply of outside resources and shipments.  As such, the goal of the British Royal Navy was to maintain these supply lines, while the German Navy attempted to sink transport ships as a means of hurting the British war effort.  Historians estimate that as many as 5,000 transport or merchant ships were lost to German U-Boat attacks in World War I.  The British, and their allies, responded by grouping merchant ships into convoysGroups of merchant ships sailing together, protected by Royal Navy warships against submarine attack. that were defended by ships from the British Royal Navy.​

“

There is such a thing as a man being too proud to fight. There is such a thing as a nation being so right that it does not need to convince others by force that it is right.

Woodrow Wilson

President of the United States, May 10th, 1915

Wilson spoke these words in Philadelphia on May 10th, 1915, three days after the German submarine U-20 sank the passenger liner Lusitania off the coast of Ireland on May 7th, 1915, killing nearly 1,200 people, including 128 Americans. The quote shows that even after the attack, Wilson chose to keep the United States out of the war, a decision that German unrestricted submarine warfare would later undo.

Source: Library of Congress© factsthatcrunch.com

Significance of German Submarines in World War I

Historians consider the German U-Boat Campaign to be one of the main reasons that the United States entered the fighting of World War I. The war caused American trade to increase dramatically to both Britain and France but the British naval blockadeA military effort to cut off a country’s access to outside trade and supplies by controlling surrounding waters. against Germany caused American trade to the Central Powers to decrease.  Ultimately, this could not be tolerated by the Germans and Germany responded by carrying out submarine warfare in the North Atlantic against the British navy and merchant ships.​

In all, the use of submarines in World War I was significant and the German U-Boat Campaign played an important role in the war.  In total, Germany built 373 U-Boats in World War I and lost 178 of them to enemy action.  However, historians estimate that the German U-Boats sank over 5,000 Allied merchant ships along with numerous other naval ships, such as battleships.  Furthermore, it is estimated that as many as 15,000 Allied sailors died as a result of German submarine warfare.​

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How did German U-Boat warfare pull the United States into World War I?

American Trade GrowthBritish Naval BlockadeGerman U-Boat Campaign, 1914 to 1918Approximately 5,000 Merchant Ships SunkSinking of the Lusitania, May 7th, 1915American Lives Lost at Sea

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The United States entered World War I in April 1917, with the German U-Boat campaign widely seen by historians as a primary cause.

American Trade Growth During World War I, American trade with Britain and France grew quickly. Germany could not accept this flow of supplies reaching its enemies across the Atlantic.

British Naval Blockade Britain blockaded German ports to cut off outside trade and supplies. This forced the United States to reduce trade with Germany, pushing American economic ties firmly toward the Allied side.

German U-Boat Campaign, 1914 to 1918 Germany launched a sustained submarine campaign against Allied merchant shipping routes across the North Atlantic. Germany built 373 U-Boats to carry out this effort over four years.

Approximately 5,000 Merchant Ships Sunk German U-Boats sank an estimated 5,000 Allied merchant and transport ships during the war. Approximately 15,000 Allied sailors died as a result of these attacks.

Sinking of the Lusitania, May 7th, 1915 The German submarine U-20 sank the passenger liner Lusitania off the coast of Ireland. Among the 1,198 people who died were 128 Americans, a fact that raised strong opposition to Germany among the American public.

American Lives Lost at Sea The repeated loss of American lives aboard ships targeted by U-Boats raised pressure on the U.S. government to respond. Germany's refusal to stop unrestricted submarine warfare left diplomatic options exhausted.

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American Trade GrowthDuring World War I, American trade with Britain and France grew quickly. Germany could not accept this flow of supplies reaching its enemies across the Atlantic.

British Naval BlockadeBritain blockaded German ports to cut off outside trade and supplies. This forced the United States to reduce trade with Germany, pushing American economic ties firmly toward the Allied side.

German U-Boat Campaign, 1914 to 1918Germany launched a sustained submarine campaign against Allied merchant shipping routes across the North Atlantic. Germany built 373 U-Boats to carry out this effort over four years.

Approximately 5,000 Merchant Ships SunkGerman U-Boats sank an estimated 5,000 Allied merchant and transport ships during the war. Approximately 15,000 Allied sailors died as a result of these attacks.

Sinking of the Lusitania, May 7th, 1915The German submarine U-20 sank the passenger liner Lusitania off the coast of Ireland. Among the 1,198 people who died were 128 Americans, a fact that raised strong opposition to Germany among the American public.

American Lives Lost at SeaThe repeated loss of American lives aboard ships targeted by U-Boats raised pressure on the U.S. government to respond. Germany's refusal to stop unrestricted submarine warfare left diplomatic options exhausted.

Source: Library of Congress© factsthatcrunch.comMore World War I Facts →

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MLA: Beck, Elias. “Submarines in World War I: A Detailed Summary.” HistoryCrunch, 29 September 2026, https://historycrunch.com/submarines-in-world-war-i/.

APA: Beck, E. (2026). Submarines in World War I: A Detailed Summary. HistoryCrunch. https://historycrunch.com/submarines-in-world-war-i/

Chicago: Beck, Elias. “Submarines in World War I: A Detailed Summary.” HistoryCrunch. September 29, 2026. https://historycrunch.com/submarines-in-world-war-i/

Sources

• Imperial War Museum
• Margaret MacMillan, The War That Ended Peace.
• Fordham Modern History Sourcebook
• Hew Strachan, The First World War.