In the First World War my two grandfathers were on opposite sides.
My mother's father William was on the side of the English. Born in Bermondsey as the oldest of ten, he enlisted at the age of 19 at the very start of the war and fought at the battles of Ypres, the Somme and Paschendale - the latter where he was badly wounded. He was in the Royal Artillery and had a horse called Smiler that used to pull the gun carriages. The horse was also badly injured and had to be put down. My grandfather luckily was brought back to England with injuries to his leg, head and eye. He lost his eye and from then on had a glass one. He was plagued by bad headaches for the rest of his life too, whenever the schrapnel moved around.
My German grandfather in exact centre front row.
When my mother and father got together in the Second World War, announced their engagement and introduced both sets of parents to one another, my two grandfathers amicably shook hands and joked "I bet you were the b*****r that shot my leg/eye". It was good that they could be so forgiving, given the extent of their injuries.