The series is a historical drama depicting the period of history surrounding the violent transformation of the Roman Republic into the Roman Empire; a change driven by civil warfare between radical populares and conservative optimates, the decay of political institutions, and the actions of ambitious men and women. The HBO website provides the following introduction:
Half a century before the dawn of Christianity, Rome has become the wealthiest city in the world, a cosmopolitan metropolis of one million people — epicenter of a sprawling empire. Founded on principles of shared power and fierce personal competition, the Republic was created to prevent any one man from seizing absolute control. It is a society where soldiers can rise up from provincial commoners to become national heroes, even leaders of the Republic. But as the ruling class became extravagantly wealthy, the foundations have crumbled, eaten away by corruption and excess, and the old values of Spartan discipline and social unity have given way to a great chasm between the classes.
While chronicling the lives and deeds of the rich, powerful and "historically significant," the series also focuses on the lives, fortunes, families and acquaintances of two common men: Lucius Vorenus and Titus Pullo, two Roman soldiers mentioned historically in Caesar's Commentarii de Bello Gallico. The fictionalized Vorenus and Pullo manage to witness and often influence many of the historical events presented in the series.
Season 1 depicts Julius Caesar's civil war of 49 BC against the traditionalist conservative faction in the Roman Senate, his subsequent rise to absolute dictatorship over Rome and his eventual fall, spanning the time period from the end of his Gallic Wars (52 BC or 701 ab urbe condita) until his assassination on 15 March 44 BC (the infamous Ides of March). Against the backdrop of these cataclysmic events, we also see the early years of the young Octavian, who is destined to become the first Emperor of Rome, Augustus.
Season 2 chronicles the power struggle between Octavian and Mark Antony following Caesar's assassination, spanning the period from Caesar's death in 44 BC to Octavian's final victory over Antony at Actium in 31 BC.