That's not a simple answer to give (so I'm not going to 🤨at @realDonaldTrump for not understanding why the US entered the war on the side of the British-French-Italians)
In April 1917, the US didn't yet have an army to send (US army was just a little over 100,000 personnel, compared to armies with millions of troops in Europe).
army.mil/article/185229…
loc.gov/collections/st…