If Caius Julius Caesar remains, after 2,000 years, one of the most famous men who ever lived, it is not simply because he was a hero, but because his life and career seem designed to demonstrate the essential ambiguity of heroism. Caesar was a brilliant general, a clement victor, and the savior of Rome; change your angle of regard by just a hair’s breadth, and Caesar was a ruthless self-seeker, a brutal imperialist, and the man who dealt the coup de grace to Rome’s ancient republican institutions. Both views were current during his lifetime, which is why he could be simultaneously the most popular man in Rome and the man whom the most patriotic Romans felt it necessary to assassinate. And the contradictions have persisted down the centuries, making Caesar a litmus test for every era’s feelings about glory, loyalty, and power.

For Dante, Caesar’s assassin Brutus was an arch-traitor, who belonged with Judas in the lowest circle of Hell. For Shakespeare, on the other hand, he was “the noblest Roman of them all, “forced to preserve his ideals by murdering a man he loved. As both of those poets knew, how we judge Caesar is of more than merely historical interest. For he represents the apotheosis of one of Western culture’s most cherished values — individual achievement, the imposition of one’s self on the world by sheer exercise of will. Indeed, no one has ever made his name in a more literal sense than Julius Caesar. When he was born, “Caesar” was the name of an old but minor Roman family; 2,000 years later, the emperor of Germany was called “Kaiser” and the Emperor of Russia “Tsar,” both versions of a word that had become a synonym for sovereignty.

Adrian Goldsworthy’s new biography, “Caesar: Life of a Colossus” (Yale University Press, 583 pages, $35), succeeds in capturing all the drama and complexity of this best-known of lives. Mr. Goldsworthy, a prolific young British classicist, has real narrative gifts, as well as an encyclopedic knowledge of late republican Rome.Together, these strengths make “Caesar” one of the most fascinating biographies you will come across this year.This is a considerable feat when writing about a man who is known to us almost entirely through a handful of ancient texts, which have been endlessly interpreted through the centuries.

The most important of those texts is Caesar’s own “Commentaries” on the Gallic and Civil Wars, whose opening words — Gallia est omnis divisa in partes tres, or “All Gaul is divided into three parts” — served as an introduction to the Latin langauge for generations of schoolboys. But the “Commentaries,” while an invaluable textbook of military strategy (Napoleon, for one, studied them carefully), tell us little about Caesar the man, and in any case were intended to show their author in the best possible light. There are a few other contemporary sources for Caesar’s life, such as the letters of Cicero, the politician, orator, and philosopher, whose career spanned Caesar’s rise and fall. But the fullest and best-known biographies of Caesar, in Plutarch’s “Lives” and Suetonius’s “Twelve Caesars,” were written a century after his death.

As a result, the Julius Caesar we come to know in “Caesar” is not the private man, but the public figure. As Mr. Goldsworthy writes, we don’t really know what Caesar looked like, or even exactly when he was born, though 100 B.C. is the scholarly consensus. He wrote many letters, at least some of which were published after his death, but none survived. We know he had three wives and dozens of mistresses, including Cleopatra, but we have no testimony from any of them about Caesar the husband, the father, or the lover. Novelists may try to supply these missing portraits, but for a historian like Mr. Goldsworthy, writing about Caesar really means writing about the Roman world in which he lived — above all, its political and military culture.

Yet this restriction is peculiarly appropriate for Caesar, whose life was shaped by his overwhelming ambition for political supremacy and martial conquest. One famous story has Caesar, in his early 30s, feeling depressed before a statue of Alexander the Great, who by that age had conquered the known world. Before he died in 44 B.C., Caesar too would have become a legendary general, adding Gaul to the Roman Empire and winning epic battles on three continents.Yet while the names of Caesar and Alexander are inescapably joined — thanks in part to Plutarch, who treated them together in his “Parallel Lives” — Mr. Goldsworthy shows that the comparison is misleading.

Alexander, one might say, represents romantic ambition, the unappeasable desire for more and more conquest and glory, which can never be satisfied on earth. For him to die young was no tragedy, but an aesthetically appropriate conclusion to a career that could only be meteor-like. Caesar’s ambition, by contrast, was deeply classical, in the sense that the rewards he sought were tangible and adequate to his desire. They were, in fact, the same prizes that all the men of his class were raised to seek: prestige and authority in Rome, and all the titles and honors that went along with them. Caesar did not want to be a living god, at least at the beginning of his career; he wanted to be elected consul, the highest office in the Republic. For the first four decades of his life, he followed the traditional path of a Roman senator, serving in the junior posts of quaestor, aedile and praetor, and cementing his political position through judicious marriages and copious bribery.

In the year 59 B.C., Caesar achieved his goal, serving a controversial year as consul, during which he passed land-reform legislation and feuded constantly with his co-consul. The next year he was sent off to govern a province, the traditional reward for a chief magistrate after his term expired. It was in Gaul during the next nine years that Caesar, formerly just a successful politician, became a world-famous general, thanks to his astonishing energy, resilience, and strategic abilities. Yet his Roman patriotism or chauvinism was never more evident than when Caesar was away from Rome. Conquering the equivalent of modern France meant nothing to him in itself; he valued his Gallic victories only insofar as they would win him prestige back home. Even on his most arduous campaigns, Mr. Goldsworthy shows, Caesar was constantly writing letters to his fellow senators, keeping his political connections alive. The “Commentaries” themselves were most likely written as propaganda, meant to remind the Romans what their proconsul in Gaul had accomplished each year.

That is why the famous crossing of the Rubicon represents such a crucial turning point in Caesar’s life, as well as in the history of Rome. When Caesar had pacified all of Gaul, he wanted to return to Rome to serve another term as consul. But knowing that he had incurred the jealousy of most of the state’s leading men — especially Pompey the Great, hitherto Rome’s most famous general — he refused to disband his army before he could be elected consul, which would give him immunity from politically-inspired prosecution. His stubbornness was matched by that of Pompey, who gained the support of the many senators who feared Caesar as a threat to the traditional Republican constitution. By taking his army across the Rubicon, the river that marked the boundary between his province and Italy proper, Caesar deliberately made himself an outlaw.

Yet even though he was willing to defy the Republic, he did not want to destroy it. Over the next five years, as Caesar crushed Pompey at the battle of Pharsalus, pacified Egypt and Asia Minor, and stamped out the last resistance in Spain, he ostentatiously refused to take vengeance on his foes. When Pompey’s assassins sent Caesar his rival’s severed head, hoping to curry favor, he wept; when Cato committed suicide rather than surrender, Caesar complained that he hadn’t gotten the chance to pardon one of his most vocal critics. Fighting Romans, for Caesar, was different from fighting Gauls. He didn’t want to reduce his fellow citizens to servitude, just to make them acknowledge his primacy.

Yet when Caesar came back to Rome at the conclusion of the Civil Wars, that primacy was too great for his fellow senators to endure. As Mr. Goldworthy repeatedly emphasizes, a Roman nobleman’s career was a perpetual struggle for prestige, and when Caesar engrossed the honors of the state — becoming dictator for life, wearing a royal diadem, even being worshipped as one of Rome’s many gods — he became intolerable. When Brutus and Cassius assassinated him in the name of liberty, Mr. Goldsworthy reminds us, what they really wanted was “a return to the dominance of a few well-established families, and the opportunity to … make fortunes by exploiting the inhabitants of the provinces.”

But the republican system was too sick to be revived, and Caesar’s murder only unleashed a new round of civil war, which ended with the establishment of his adopted son, Augustus, as the first Roman Emperor. Julius Caesar’s tragedy, Mr. Goldsworthy shows, was that the only way he could become the greatest Roman was to destroy the Roman Republic. In doing so, he exposed the paradoxical nature of all worldly ambition, which must both exalt and humiliate, create and destroy. That is why Caesar’s life is still one of our culture’s most potent and disturbing parables.