Lusitania: The sinking of the Lusitania by a German U-boat in 1915 shocked the world as over 1,100 civilians died, including Americans. This event contributed to the eventual entry of the United States into World War I.
Blockade: The British naval blockade on Germany restricted supplies from reaching its shores during WWI. This led to shortages of food and resources within Germany and contributed to social unrest and economic hardships.
Submarine Warfare Policies: Prior to unrestricted submarine warfare, Germany had employed various submarine warfare policies such as cruiser rules (warning before attack) and prize rules (allowing searches before sinking). These policies aimed to limit civilian casualties but eventually gave way to unrestricted attacks due to strategic considerations.