In early 1915, Germany had a policy of unrestricted submarine warfare which meant they could torpedo and sink armed merchant ships but not passenger ships. They broke this policy on march 24th 1916 by attacking a French cross-channel passenger ferry called the Sussex. 50 people died and even though no US citizens were killed, the attack caused president Woodrow Wilson to declare that if Germany continued with this behavior, the US would break diplomatic relations with them. In 1915, the first American merchant ship was sunk due to German aggression. The William P. Frye was attacked off the coast of Brazil on January 28th by a German cruiser which made the US angry.
On May 4, 1916, Germany tried to win over the US by proposing the Sussex Pledge.
On May 4, 1916, Germany tried to win over the US by proposing the Sussex Pledge.