We discover the story of the Twin Cities' forgotten, but undeniably first, NFL franchise(s) with the help of football writer/historian R. C. Christiansen ("Mill City Scrum: The History of Minnesota's First Team in the National Football League").

From the "Mill City Scrum" book jacket:

"In the flour milling city of Minneapolis, Minnesota, a group of first-generation American teenagers team up to play football in the sandlots. They call themselves the Marines, and with no high school or college experience, they learn to dominate their opponents using the same offense as the University of Minnesota Gophers.
"The Marines later emerge as an independent professional team, and they claim city, state, and regional championship titles, but World War I sends Marines players across the globe. When they return, the Marines face player defections, bad publicity, and low fan support.
"A former player and team captain and the manager of the Marines decides to bet his own fortune and the team’s future on a new National Football League. The Minneapolis Marines, later named the Minneapolis Red Jackets, play six years in the NFL and leave their mark on the history of pro football in Minnesota.

"Long before Minnesota had the Minnesota Vikings, they had the Minneapolis Marines."