Greetings! This is certainly one of those "big questions" which I imagine have been asked about (or at the very least, more specific aspects of American intervention in either war) on AH before. This previous response of mine regarding the American entry into the First World War should be of use, alongside this one on a similar question. For a tad more context about what America was up to before 1917, and how it factored into the whole picture of the Great War, this lecture by Michael S. Neiberg ought to cover some good ground.

Note: Though the US formally entered the war on the side of the Entente powers in April 1917, it was by no means "isolationist" up until then. u/IlluminatiRex and myself delve into how involved America was with the Great War before 1917 in this thread here.

Feel free to ask any follow-ups regarding what is mentioned here as well, and hope this helps!