Page 26. " 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.

Page 35. " a hussar of the Life Guards "

 Hussars are a type of light cavalry regiment originating in medieval Hungary. The Russian hussars were formed in 1707 under Peter the Great, but became a prominent force in the Russian Army during the Napoleonic Wars, by which time they formed ten regiments.

Page 39. " celebrated beau of Catherine the Great's day "

 Catherine the Great (1729-1796) ruled as Empress of Russia from 1762-1796. During her reign Russia became one of the most powerful empires in Europe.

Born as Sophia von Anhalt-Zerbst-Dornburg in Stettin, Germany, Catherine rose to power after her eccentric husband Tsar Peter III was assassinated six months into his reign. Despite her German origin, Catherine succeeded the throne, following the precedent set by Empress Catherine I in 1725. Although she had many lovers during her reign, most notably Grigori Alexandrovich Potemkin, she never openly remarried, retaining her independence and power over the Russian Empire. Her reign saw the expansion of the Russian Empire and of its power in Europe, as well as a flowering of art and culture within its borders. Catherine was a proponent of the Enlightenment, and took a serious interest in liberal Western European philosophies, corresponding with the French philosophers Diderot and Voltaire. She applied these ideas in modernizing aspects of the Russian government, although she failed to extend her liberalism to the issue of serfdom in Russia.

A good short biography of the empress is Catherine the Great (Life & Times) by Michael Streeter, pub. Haus Publishing.