The Sussex Pledge was a promise made by Germany to the United States in 1916. It was broken on March 24 1916, when a German submarine torpedoed the ferry S.S. Sussex which killed fifty and injuring hundreds, including three Americans.
On May 7, 1915, the British ocean liner RMS Lusitania, which primarily ferried people and goods across the Atlantic Ocean between the United States and Great Britain, was torpedoed by a German U-boat and sunk.
The sinking of the Lusitania enraged Americans and hastened the United States' entrance into World War I.
Unrestricted submarine warfare is a type of naval warfare in which submarines sink vessels such as freighters and tankers without warning, as opposed to attacks per prize rules
Germany broke its promise to the U.S. to stop attacking passenger ships, angering the U.S.
Great Britain was a leading Allied Power during the First World War of 1914–1918, fighting against the Central Powers, especially Germany.
Britain, however, was one of America’s closest trading partners, and tension soon arose between the United States and Germany over the latter’s attempted quarantine of the British Isles
The Zimmerman Telegram was a secret diplomatic communication issued from the German Foreign Office in January 1917 that proposed a military alliance between Germany and Mexico.The Zimmerman Telegram helped turn the U.S. public, firmly against Germany. On April 2, President Wilson, who had initially sought a peaceful resolution to World War I, urged immediate U.S. entrance into the war.