On May 7, 1915 a German U-boat sank the Lusitania. 1200 people drowned including 128 Americans.
Germany announced that it would attack and sink all ships, even passenger ships, that tried to sail to Britain. Germans knew this would upset the United States. There were strong political and economic ties with Britain, however, Germany wanted to stop ships from carrying food and supplies hoping it would cause Britain to give up.
British leaders told president Wilson about a secret German telegram they had decoded. The telegram was sent by foreign Secretary Arthur Zimmerman to a German representative in Mexico. Germany asked Mexico to go to war with the United States if the US entered World War 1. If Germany won Mexico would get money and reign of the lands it has lost to the United States.
In March 1917 German U-boats sand 5 US merchant ships.
After being pushed my Germany the United States enters the European War.
After five years of battle the Allies take victory over the Central Powers.