On August 9, 48 BC — the same calendar date, by a coincidence historians have noted, as the Battle of Adrianople 426 years later — Julius Caesar’s army defeated Pompey’s at Pharsalus in Thessaly, ending the civil war between them in a single afternoon and ending the Roman Republic as a functioning political institution in any meaningful sense. The Republic would survive in form for another seventeen years, until Augustus completed its constitutional conversion. But Pharsalus was where it ended in fact, because Pharsalus eliminated the only man with the political authority, military reputation, and institutional support to contest Caesar’s supremacy on terms the existing system could legitimate.