At first, the US was not interested in joining WWI.
But when they were doing trade with England and Germany, the opposing country would destroy the ship and take the supplies.
Woodrow Wilson, the US President, said that any American deaths in submarine attacks would be blamed on Germany.
Germany ignored what Wilson said and they went on to sink the British passenger liner called the Lusitania in 1915 and many Americans died.
In 1917, the British intercepted a telegram sent from Germany to Mexico. The Germans planned to make an alliance with Mexico in the case that the US entered the war. This telegram was known as the Zimmermann Telegraph and was published in the front pages of US newspapers
We intend to begin on the first of February unrestricted submarine warfare. We shall endeavor in spite of this to keep the United States of America neutral. In the event of this not succeeding, we make Mexico a proposal or alliance on the following basis: make war together, make peace together, generous financial support and an understanding on our part that Mexico is to reconquer the lost territory in Texas, New Mexico, and Arizona. More on page 19.
Zimmermann Telegram confirms Germany's alliance with Mexico!
All of this angered Wilson, so in the end, they started to fight on the Allies' side in the Great War.