When I first met the gal who would become my wife, we were sitting one evening looking through her family scrapbook. She said, "Here's a pic of my grandfather." I did a double-take and thought, 'Wrong army!' That's him on the right in the pic below, pulling guard duty in the helmet with the rifle over his shoulder. His name was Johann Dietzen VI. The photo was taken at Wartburg castle in 1916. The castle was built during the middle ages and overlooks Eisenach, his home town. Though wounded, he survived WWI and brought his family to the US from Germany in 1931. His son, my wife's father, was named Johann Dietzen as well, Americanized to John.
Wartburg castle is the place where Martin Luther translated the New Testament of the Bible into German.
My paternal grandfather was a US cavalryman on the Mexican border in 1916. He rode with Pershing's Punitive Expedition into northern Mexico in 1917. He didn't serve in Europe, so fortunately the two men never got the chance to shoot at each other!