A previous article has discussed the approximately 40,000 American who served with the Canadians in WW1, and another has featured an American in the Foreign Legion. There were also a few Americans who served in the British army.
One of these was 2nd Lieut. Henry Augustus “Harry” Butters, RFA (1892 – 1916), who was killed during the Battle of the Somme in 1916. You can read his full story here.
After the war a volume of his letters was published, and you can read the digitized version here.