Brutus was probably born in 85 b.c. of a consciously tyrannicidal family, which claimed descent on the father's side from L. Junius Brutus (who overthrew the Tarquins in 509 b.c.) and on the mother's, from C. Servilius Ahala (who slew the aspiring dictator Spurius Maelius in 439 b.c.). These claims seem fanciful: the family cannot with certainty be traced back beyond the late fourth century b.c. After Pompey the Great's treacherous slaughter of Brutus's father in 77 b.c., Brutus was adopted by his maternal uncle, Q. Servilius Caepio, and hence was often referred to as Caepio Brutus.
The young man first appeared in Roman life during the so-called First Triumvirate, the political coalition of Caesar, Pompey, and Marcus Crassus that was formed in 60 b.c. Brutus was accused, falsely, of plotting Pompey's murder (59 b.c.) and soon thereafter accompanied another uncle, M. Porcius Cato, into virtual exile on Cyprus (58 b.c.), a province where, later if not then, the nephew had usurious investments. Brutus went eastwards again--possibly in connection with his money-lending operations--in 53b.c. with his father-in-law, Appius Claudius, governor of Cilicia in Asia Minor.
When the civil war between Caesar and Pompey broke out (49 b.c.), Brutus, to the surprise of everyone, joined his father's murderer, Pompey, doubtless following Cato's example. Brutus distinguished himself at Dyrrhachium, the battle on the Adriatic coast of what is now Albania; after Pompey's defeat at Pharsalus in north-central Greece, Brutus was spared by Caesar and subsequently received various distinctions. The future assassin became governor of Cisalpine Gaul (46 b.c.), served as urban praetor (44 b.c.), was promised the governorship of Macedonia, the province north of Greece, for 43 b.c., and was marked out for the consulship. Despite these tokens of Caesar's favor, he fell in with Gaius Cassius's proposal to murder the great dictator, and indeed came to be regarded as the real leader of the plot. Brutus's part in the murder has been immortalized by the tradition that Caesar died manifesting reproachful surprise at Brutus' participation.
After Mark Antony's inflammatory speech at Caesar's funeral, the leaders of the conspiracy prudently withdrew from Rome. By September 44 b.c. Brutus was in Athens. From there he moved north into Macedonia, the province Caesar had designated as Brutus's. Its retiring governor, the son of the orator Quintus Hortensius, recognized Brutus's claim and handed over the province and its powerful army. Brutus also soon won over the troops in the adjoining province of Illyricum.
Meanwhile Antony had induced the senate to allot Macedonia to himself, or rather to his brother Gaius. The latter, however, when he crossed the Adriatic was met by Brutus's forces at Apollonia, besieged, and finally captured (March 43 b.c.). Thereupon the senate pronounced Brutus the legitimate governor of Macedonia and, after Antony's defeat at Mutina in north-central Italy (April 43 b.c.), even named him, jointly with Cassius, commander-in-chief throughout the eastern provinces. Brutus, with an eye to booty, next conducted a campaign against a Thracian tribe; but when, in November 43 b.c., Antony, Octavian (later the Emperor Augustus), and Marcus Lepidus formed the Second Triumvirate, Brutus, knowing that he would have to wage war against the new coalition, proceeded to Asia Minor to raise men, ships, and money and to join forces with Cassius. Precious months were lost plundering Lycia on the mainland and Rhodes, the island off the coast, for a war chest; and it was not until late in 42 b.c. that Brutus and Cassius headed westwards. They encountered Antony and Octavian in Macedonia, and the double battle of Philippi ensued. In the first action Brutus defeated Octavian. Cassius, fearing all was lost, committed suicide. In the seccond action, about three weeks later, Brutus was worsted, whereupon he, too, killed himself (Oct. 23, 42 b.c.).
Brutus, though often depicted as a man of principle who fought for republican liberty while opposing unnecessary bloodshed, was hardly, in Shakespeare's phrase, "the noblest Roman of them all." A typical senatorial aristocrat, he rigidly upheld the privileges and vested interests of the nobiles, the traditional governing class. His brutal treatment of provincial communities and his acceptance, while unqualified, of a provincial governorship emphasize how he assumed that men of his class had the right to rule and to exploit the state apparatus for their own advantage. What he found intolerable was one man's becoming all-powerful. Brutus, a studious, bookish man, for whom the great orator, writer, and statesman M. Cicero named and dedicated major treatises, no doubt could find yet other arguments to justify his bloody deed. Greek philosophy advocated the murder of tyrants, and Caesar's seduction of Brutus's mother supplied a personal motive. But these considerations were secondary; Caesar's real offense lay in becoming dictator perpetuus, dictator for life. Undoubtedly much influenced by his uncle Cato, whom he greatly admired (as is attested by Brutus's divorce of Claudia to marry his uncle's daughter and by the panegyric the nephew composed on Cato), Brutus developed the single-minded conviction that supremacy should belong to the senatorial class as a whole, and not to a single individual. As Brutus himself put it: "I shall fight any power that would put itself above the law."