Final answer:
Germany adopted submarine warfare during WWI primarily due to the British economic blockade, to maintain naval supremacy, and for a military advantage. The use of U-boats was a reaction to the inability to import goods and to counteract British and Allied control of resources by targeting supply ships.
Step-by-step explanation:
Three primary reasons that the German government resorted to submarine warfare during World War I were an economic blockade by the British, the need to assert naval supremacy, and to gain a military advantage.
The British blockade of German ports severely restricted Germany's ability to import goods, leading to shortages felt by every German family. To retaliate and disrupt the British economy, Germany utilized submarines as it could not enforce a similar blockade with surface ships. The unrestricted submarine warfare declared by Germany targeted ships in the war zones, sinking vessels carrying supplies to Britain and France, countering the flow of resources to the Allies.
Germany also expanded its submarine fleet from 24 to 120 by 1917, declaring the sea-lanes to Europe and France as war zones. This strategic move aimed to cut off the lifeline of cargo coming from the United States, forcing Britain to potentially withdraw from the war due to material shortages, despite the risk of bringing the U.S. into the conflict.