Question

What caused the United States to enter world war 1 after three Years of neutrality

Answer 1

There are many reasons for the US to enter world war 1, including:

Unrestricted warfare: The german submarine fleet continued to sink US and other neutral ships that had supplies to send to the Allies to try to cut off Allied material power.
Sinking of Lusitania: With 128 Americans on board, the Germans sank the Lusitania, which caused outrage among the US citizens. The Germans had to stop unrestricted warfare for awhile. However, with Germany losing, they had to change to a more drastic measure, and continued it.
Zimmermann telegram: The last straw to the US, in which the German prime minister (?) sent a telegram to Mexico, asking it to join Central power side, in exchange for help to take back all the and they lost to the US during the Mexican-American war.

hope this helps