My paternal grandfather served in WW I, too — on the German side. He was on the eastern front and was captured by the Russians and spent quite a long while as a Russian POW. He escaped, walked back to Germany, and then took his wife and young family to New York in 1927. His two sons served for the US in the next world war.
Same with my maternal grandfather. Only he wasn't smart enough to move to the US. So he got a "Uncle Adolf wants you for military service" notification.
My Father was a combat veteran of WWII. They landed at Normandy and became part of the Army of Occupation in Berlin.
He once told me both his sergeant and lieutenant were full blooded Germans.