The United States entered the WWI because Germany, in the east, were expanding their power and also decided to resume their unrestricted submarine warfare which was also known as 'Zimmerman's Telegram'.
Additional Points: After the WWI, the US intervened and bailed Germany out of the Hyperinflation crisis in Germany.
Then were affected by the Great Economic Depression from 1929-1932.
That is why they remained neutral until Japan forced them to enter the WWII by bombing US' naval base, Pearl Harbour.
Hope this helps you!
