Frank Tannen was a Sergeant in the United States Army and was stationed in Hill Valley in 1944. Marty McFly and Verne Brown encountered Sgt. Tannen after traveling to the past to find the lost blueprints for a machine that Doc had invented.
Sometime after 1944, Frank no longer lived in Hill Valley, and by 1955, Biff lived with his Grandmother Tannen. Biff stated that he had an uncle who abandoned him, so it's likely that Frank left Hill Valley at some point before 1955.
Behind the scenes
- In the episode, Frank uses German expressions twice, calling Marty a "weisenheimer" and a "dummkopf", and corrects himself both times. However, his conduct as a sergeant in the United States Army does not appear to show allegiance to any other country.