Unrestricted submarine warfare: Germany's decision to resume unrestricted submarine warfare was a major factor in the United States' decision to enter the war. Germany had previously pledged to restrict its submarine attacks to military targets, but in early 1917 it began sinking merchant ships without warning, regardless of their nationality. This policy caused the deaths of hundreds of American civilians and merchant seamen.
Zimmermann Telegram: Another factor that contributed to the United States' entry into the war was the Zimmermann Telegram. The telegram was a secret message from the German Foreign Secretary, Arthur Zimmermann, to the German ambassador in Mexico, Heinrich von Eckardt. In the telegram, Zimmermann proposed an alliance between Germany and Mexico in the event of war between the United States and Germany. The telegram was intercepted and decoded by British intelligence, and its publication in the United States caused widespread outrage.
Sabotage in the United States: In the months leading up to the United States' entry into the war, there were several incidents of German sabotage in the United States. These included the bombing of the Black Tom munitions depot in Jersey City, New Jersey, and the sinking of the passenger ship Lusitania. These incidents contributed to the growing anti-German sentiment in the United States and made it more difficult for the United States to remain neutral.
In conclusion, the United States entered World War I in April 1917 due to a combination of factors, including Germany's resumption of unrestricted submarine warfare, the Zimmermann Telegram, and incidents of sabotage in the United States.