"it was Caesar's Commentaries"
Julius Caesar
Public DomainJulius Caesar

 Julius Caesar (100-44 BC) was Consul of the Roman Republic from 49-44 BC, during which time he oversaw the shift of Rome from republic to empire. A gifted general, Caesar's military campaigns included the conquest of Gaul and victory in the Civil War of 49–45 BC, which he described in his Commentaries on the Gallic War and Commentaries on the Civil War. He was a gifted writer and public speaker. Caesar was assassinated by a group of Roman senators on the Ides of March (15th March), as described in Plutarch's Life of Caesar:

But those who had prepared themselves for the murder bared each of them his dagger, and Caesar, hemmed in on all sides, whichever way he turned confronting blows of weapons aimed at his face and eyes, driven hither and thither like a wild beast, was entangled in the hands of all; for all had to take part in the sacrifice and taste of the slaughter. Therefore Brutus also gave him one blow in the groin. And it is said by some writers that although Caesar defended himself against the rest and darted this way and that and cried aloud, when he saw that Brutus had drawn his dagger, he pulled his toga down over his head and sank, either by chance or because pushed there by his murderers, against the pedestal on which the statue of Pompey stood.

The Russian word 'tsar' comes from the Latin 'caesar'. Tolstoy's reference to Julius Caesar gives the reader an oblique insight into Pierre's early ambitions. An earlier draft of War and Peace makes this clearer: "Pierre dreamed of being [...] a commander of the type of Caesar or Napoleon." Tolstoy later cut that line out and replaced it with this far more subtle allusion.