Then there was the case of the Lusitania, a British passenger liner, that was sunk because it was supposedly mistaken for being a ship carrying weapons. On the ship, there were 128 Americans that were killed because of the attack. With all of the hype of the German U-Boats and how uncivilized their attacks were, it was then when the Sussex Pledge came into action, after sinking the Allied Ship called the Sussex, in which Germany would have to promise to warn other ships that they were going to attack them.
Also, the Zimmerman Telegram played a factor into leading the U.S. to join the Allied forces in WWI. The Zimmerman Telegram is where the German's foreign secretary, Arthur Zimmerman, sent a telegram to Mexico to make a secret offer of Alliance and wanted for Mexico to attack the United States. Mexico declined the offer and made relations between America and Germany worse. Then came the Russian Revolution in which Russia's autocratic leader, Czar Nicholas II, was forced to give up his power which led to a republican government to rise and replace the monarchy, leaving the United States no reason not to join the Allies. Finally, with the sinking of the Vigilancia, Illinois and the City of Memphis, all of which were American ships sunk by the Germans. This sent America over the edge and they declared war on Germany a few days after the attack. These were all contributing factors of what caused Americans to get involved in the Great War... World War I!