1 janv. 1917 - Germany’s unrestricted submarine warfare campaign begins.

Description:

In early 1917 Germany begins indiscriminately sinking enemy vessels. Freighters, Merchant ships and Warships included.
This is one of the events often claimed to be the reason for U.S involvement in the war.

Source:
History, "Germany resumes unrestricted submarine warfare"
(http://www.history.com/this-day-in-history/germany-resumes-unrestricted-submarine-warfare)