7 mai 1915 - The “Lusitania” sunk by Germany

Description:

German submarines sink British naval ship "Lusitania" off the coast or Ireland. Killing 114-128 American passengers. The ship was carrying arms and munitions.
This is one of the events often claimed to be the reason for U.S involvement in the war.

Source:
History, "German submarine sinks Lusitania"
(http://www.history.com/this-day-in-history/german-submarine-sinks-lusitania)