Firstly, I don't believe it would have caused such havoc in the United States that because one of their own citizens was executed by The British, it would have stopped them entering WW1!
My understanding is that Maxwell asked one of his advisors what should be done with this De Valera chap. The response was - he's a nobody who will never come back to haunt us!petunia
The word that he had a breakdown while in command at Bolands' Mill and had to be locked in a room by his own men, to keep order, must have got around!
How does the world change if De valera's remains were limestoned with the 1916 heroes who had been executed as well?