World War I
The German leaders decided to use submarines, called U-boats, from the German word Unterseeboot (meaning "underwater boat"). The U-boats attacked passenger ships such as RMS Lusitania carrying civilians to the United Kingdom. They did not follow the laws of war because the British could easily destroy them otherwise. America was selling weapons to Germany's enemies but not to Germany and so was not neutral since the US had taken sides during the conflict. Many American and British noncombatants were killed by the submarines although they were not fighting.