What was the first incident of unrestricted submarine warfare?

The first incident of unrestricted submarine warfare occurred during World War I when Germany declared the waters around the British Isles a war zone on February 4, 1915. This policy allowed German U-boats to target not only military vessels but also civilian ships without warning. The most notable incident was the sinking of the RMS Lusitania on May 7, 1915, which resulted in the deaths of 1,198 passengers, including 128 Americans, and sparked outrage that contributed to the U.S. entering the war. Unrestricted submarine warfare was a significant factor in changing naval warfare and international relations during the conflict.