The Republic did not fall in a day. It was worn away by a generation of overmighty generals until one man stood alone, and then his heir dressed the monarchy in republican clothes.
The First Triumvirate
By the 60s BC, three men overshadowed the state: Pompey, Rome's greatest general; Crassus, its richest man; and Julius Caesar, its most gifted politician. In 60 BC they formed a private pact, the First Triumvirate, to run Rome between them, bypassing the Senate. Caesar took command in Gaul and, over eight years of conquest, built a devoted, battle-hardened army and a fortune to rival Crassus's.
Crossing the Rubicon
The alliance frayed. Crassus died on campaign; Pompey drifted to the Senate, which ordered Caesar to give up his army and return as a private citizen: to certain ruin. In 49 BC Caesar chose war instead, leading a legion across the Rubicon, the little river marking the edge of Italy, an act of open rebellion. "The die is cast," he is said to have remarked. In the civil war that followed he beat Pompey and made himself master of Rome.
The Ides of March
Caesar had himself named dictator for life and gathered the honours of a king in all but name. To a Senate steeped in five centuries of hatred for monarchy, that was intolerable. On the Ides of March, 15 March 44 BC, a group of senators (including a Brutus who invoked his ancestor's expulsion of the kings) stabbed Caesar to death in the Senate house. They believed they were saving the Republic.
Augustus
They had killed it instead. Caesar's death brought only more civil war, ending when his young heir Octavian defeated Mark Antony and Cleopatra at Actium in 31 BC. Octavian had learned from his adoptive father's fate: he took the modest title princeps, "first citizen," left the Senate and its offices standing as a hollow shell, and let Rome believe the Republic had been restored. As Augustus, the first emperor, he ruled everything while claiming to rule nothing. The forms of the Republic survived for centuries; its substance was gone.