My two grandfathers were on opposite sides in WWI, although neither one was anywhere near the other. My Austrian grandfather was a logistics officer on the staff of a general in Italy; my English grandfather was in France in 1914-15, was sent to an officer's training course, and spent the rest of the war as an operations officer on General Allenby's staff in Egypt and Palestine (he knew TE Lawrence, and didn't care for him).

I have an ancestor who fought in the American Revolution. On the British side.