Britannica Sources used in this answer

Why did Germany's unrestricted submarine warfare anger the United States?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

World War I A British soldier inside a trench on the Western Front during World War I, 1914–18.

Germany’s use of unrestricted submarine warfare angered the United States primarily because it violated traditional maritime norms and endangered American lives. By declaring the waters around the British Isles a war zone where Allied ships would be sunk without warning, Germany abandoned established practices such as boarding, search and seizure, and the provision of safety for civilians.12

The sinking of the British ocean liner Lusitania on May 7, 1915, was a pivotal moment that outraged American public opinion, as 128 U.S. citizens were among the 1,198 people who drowned. Although Germany argued that the ship was carrying war munitions, the U.S. government viewed the attack as an indictment of the principles of submarine warfare.314

Subsequent attacks on other vessels, such as the Arabic and the Sussex, further escalated tensions. While Germany periodically restricted its submarine operations following U.S. diplomatic protests, the decision to resume unrestricted warfare in February 1917 ultimately led the United States to break diplomatic relations and formally enter World War I on April 6, 1917.1526