The NFL is nearly synonymous with America today. Practically nothing is more quintessentially and universally American than tuning in every Sunday (and Monday, and Thursday… and sometimes Saturdays and holidays too) to watch the world’s most beautiful ballet of violence. It generates the most revenue of any sports league globally and sets new records for team valuations each year. But it wasn’t always this way. The history of the NFL mirrors America’s own development: scrappy small-town teams rode the successive growth waves of the automobile, TV, the Internet and social media to grow larger than the...