If there is a single must-read for students of history, politics, warfare, and international relations, it is Thucydides’ History of the Peloponnesian War, recounting the struggle between the empires of Athens and Sparta in the last three decades of the fifth century BCE. Its modern influence reached a peak — and has remained there ever since — in the second half of the 20th century, as exemplified by Admiral Stansfield Turner’s placing it at the head of the reading list for officers at the U.S. Naval War College when he became its president in 1972. It has remained on all relevant reading lists since.

Turner was prompted to make his officer students read Thucydides by the parallels that the latter’s classic text offered. Thucydides wrote of the conflict between two great rivals who had shortly beforehand been allied against a dangerous third party — he meant Athens and Sparta against Persia; for Turner, the parallel was the U.S. and USSR against Nazi Germany. But Turner also saw a parallel between Athens’ failure in the Sicilian Expedition, launched in 415 BCE, and the bitter experience of Vietnam then still fresh in American minds. In the tensions of the Cold War it seemed to many more than Turner alone that Thucydides’ overarching view — that war is the basic condition of mankind, and that the chief motives of international action are fear and self-interest — was unimpeachably right.

It is not only the matter but the manner of Thucydides’ great work that is inspirational. He regarded the war between Athens and Sparta as the greatest and most significant that the world had seen to that date, and that the lessons it taught would therefore be of perpetual importance to mankind. (We might now attach a different weighting to the importance of the Greco-Persian war half a century earlier, in which the Battle of Salamis, 480 BCE, saved the cradle of Western civilisation from Oriental invasion.) Moreover Thucydides claimed that he wrote his history in a thoroughly scientific spirit, with neutrality and dispassion, seeking the objective truth by sifting and examining evidence and weighing the inconsistencies between different accounts of what happened. His claim to be the first-ever rigorous historian is backed by the fact that he was actually there, at the outset as a senior participant and then as an avid spectator of all that happened.

Few historians are in a better position than Donald Kagan to evaluate Thucydides’ merits and achievement, which is the task he sets himself in this new book. Kagan’s four-volume history of the Peloponnesian War, followed by a brilliant one-volume epitome of it, are the standard contemporary texts in the field, and he has parlayed the wisdom gleaned from his close study both of that war and Thucydides’ account of it into discussions of the origins of war, the possibilities of peace, and contemporary geopolitics. When therefore he argues that Thucydides’ account of the Peloponnesian War is tendentious and revisionary, and in important respects misleading, one does well to sit up and take notice. For this indeed is the burden of Kagan’s striking account, which in forensic and exacting style places Thucydides in the historiographical dock.

This does not mean that Kagan is hostile to Thucydides; not a bit of it. He is an admirer — who could not be? — but an objective one. He reveals Thucydides as a thoroughly revisionary historian, bent on opposing the view widely held in his own day that Athens’ disaster in the Peloponnesian War was the fault of Pericles, whose mistakes in foreign policy were its cause, and whose early management of it planted the seeds of defeat. Instead Thucydides wished to establish an alternative thesis: that the war was inevitable because of Sparta’s fear of Athens’ growing power, and that it was the decayed quality of Athenian democracy after Pericles, exemplified by the crudity of Cleon and other lesser men, that betrayed Athens to defeat. In the process Thucydides sought to defend the reputation not only of Pericles but also Nicias, leader of both the peace party and of the disastrous Sicilian campaign.

By careful argument Kagan puts the record straight. He notes that Thucydides, in order to shape his readers’ interpretation of events, is very selective in reporting speeches in the Athenian assembly and very economical with the facts of what happened in various battles and campaigns, such as the loss of Amphipolis (where Thucydides himself had been in command, and whose loss resulted in his exile by his fellow Athenians). Thucydides quotes only those speeches in the assembly that bear out his version of events; because he is on the whole careful and accurate in conveying the burden of what was said (apart from his own scruples, his contemporaries would have caught him out otherwise), he chose not to give the anti-Pericleans any ammunition by presenting the case made by those whose view of events he was determined to contradict.

By contrast Kagan tells us, among other correctives to Thucydides’ picture, that though Cleon might have been a vulgarian and a hawk, he was a notable warrior, and that though Nicias might have been a dove stamped in the same mould as Pericles, his incompetence in Sicily turned a defeat into a catastrophe.

Thucydides was not trying to mislead; as he saw it from his own partisan viewpoint, he was trying to correct. In doing so he was revising the standard view of the war held by his contemporaries. Kagan likewise is revising our view — not of the war but of Thucydides himself; not to impugn him, but to set the record straight by revealing the great historian’s bias and aim, and rescuing those he unfairly attacked. The case Kagan makes seems hard to fault, so carefully does he argue it and so copiously does he substantiate it; though doubtless among the scholars — whose ingenuity one should never underestimate — occasion will be found for nits to be picked.

Getting the record straight in Kagan’s terms makes very little difference to the value of Thucydides’ work as a textbook for politics and diplomacy. In these domains the intricacies of calculation explored by Thucydides, the dangers of weak allies drawing their stronger partners into conflict, the inevitability that suspicion and self-interest will exacerbate bad situations, and the ultimate fact that it is economics that wins wars, all remain starkly true. In an earlier book Kagan argued that the chief parallel between Thucydides’ war and the recent past is with the First World War, and indeed in 1914 it was entanglements of alliances, looped round spinning axes of suspicion and self-interest, that drew each party and thereby an entire civilization into the abyss. In the darkening days of 431 BCE it was the peripheral colonies of the major cities that began to skirmish, petitioning for their parent cities’ help, and the parent cities, watchful of revolt and secession in their empires, became drawn in against their will. The warning in that tale remains clarion clear today.

Kagan’s book, fascinating and characteristically lucid, is one of those little masterpieces that permanently reconfigure intellectual landscapes. Thucydides will not be the same after this, though neither diminished as a historian nor less significant as a teacher for all time. And it revivifies interest in everything it discusses; after an absence of several decades I am now back in the fifth century BCE, with both Thucydides and others of Kagan’s books in hand.