The Circus Maximus was the largest sports venue the ancient world ever built, capable of holding somewhere between 150,000 and 250,000 spectators — the ancient sources give figures that seem implausibly large but are not entirely implausible given the site’s archaeology. For comparison, the Colosseum held perhaps 50,000 to 80,000. The Circus was Rome’s dominant entertainment venue, chariot racing was Rome’s dominant spectator sport, and the passion Romans invested in the circus factions — the Blues, Greens, Reds, and Whites — was of an intensity that modern sports tribalism only partially approximates. In Constantinople, a dispute between circus factions contributed to a riot that killed tens of thousands of people and nearly ended Justinian’s reign. This is the world that chariot racing inhabited.