Thucydides of Athens was born between 465 and 460 B.C. He was in his thirties when the Peloponnesian War broke out, a twenty-seven-year war in which he took part. We do not know what campaigns he took part in during the early years of the war. In 424, he was elected strategist, i .e. magistrate in charge of commanding the armed forces. Placed at the head of the Athenian naval expedition in Thrace, his first campaign as a general turned out badly. He could not arrive in time to prevent the Spartan Brasidas from taking Amphipolis, which earned him a sentence of exile. In the absence of any testimony, it is impossible to assess his share of responsibility in this failure or even to judge his military qualities. It was during this exile that he devoted himself to his History of the Peloponnesian War. Recalled after the fall of Athens in 404 and the amnesty decreed by the victors, he returned to his home town and died shortly afterwards.Between 399 and 396 he died without having completed his work, which was interrupted in the middle of the twenty-first year of the war.

The Peloponnesian War between 431 and 404 pitted the imperialists of two cities at their height, Sparta and Athens. Thucydides, in analysing the causes and the course of the conflict, puts into practice a strategic reading, in the contemporary sense of the term, since he highlights the dialectical character of a struggle in which the protagonists transform their action in contact with the opponent's action. Victory belongs to the one who has best understood and adapted to the opponent's strategy.

Four levels of analysis can thus be distinguished in the book:

• The first, situated at the articulation of the political and the strategic, is the study of an imperialism, that of Athens. This phenomenon is, according to Thucydides, at the centre of the event: it explains the origin of the conflict, determines the general strategy adopted, freezes this strategy in a conservatism that translates into theThe result is the impossibility of renouncing the empire, and ultimately leads to defeat because of the unpopularity generated by the organization of this empire.
• The second is a geostrategic reading of the struggle between a maritime power and a continental power.
• The third is the strategy itself, through an analysis of the indirect approach, its effectiveness and its limits.
• Finally, the fourth is in the field of strategic thinking. It contains the idea that all strategy is a matter of adaptation and innovation, inertia can only lead to defeat.

Between Sparta, a continental power, and Athens, a maritime power, two strategies will confront each other. They are set out in the speeches that Thucydides gives to the leaders of each side: the Spartan Archidamos and the Athenian Pericles.

The Pericles option is an oblique strategy that combines offensive and defensive modes. The defensive manoeuvre is based on the principle that not everything can be defended or saved. Considering Athens as an island, it abandons the territory to the opponent, withdraws the population behind the city walls and places its salvation in the fleet, the Peloponnesians, i.e. Sparta and its allies, having no experience of the sea. He thus sacrifices the territory to the empire from which Athens draws its power. Thucydides then uses, for the first time in history, the expression "mastery of the sea". This manoeuvre is completed by another: to form a network of alliances to encircle the Peloponnese. In the offensive register, refusing to take a frontal approach, Pericles advocates an indirect action, with economic and psychological objectives, which consists of ravaging the cultures of the Peloponnese by sea. We are fully committed to what Liddell Hart will call the indirect approach.

The Peloponnesian strategy, on the other hand, is based on the search for the land battle, the engagement of the fort to the fort in the main theatre. And, if it also advocates the destruction of cultures, it is to force the adversary to engage. But this great hoplitic battle sought by Sparta is denied him. For ten years, overall, the strategic initiative belongs to Athens. Its raids by sea destabilized the Peloponnesians more than the ravages of Attica weakened the city of Pericles.

The Peloponnesian War is initially this struggle between two powers of radically different nature and strategy. But, better than the Athenians, the Spartans and their allies were able to adapt by building up a powerful fleet. Thus, the conflict gradually tends to become a naval war, and the two decisive battles, Syracuse and Aigos Potamos, are naval battles. These two Athenian defeats illustrate the words of Albert Thibaudet: "The sea triumphs or is defeated only by the sea".

The Peloponnesians also learned to practice indirect strategy: they attacked the opponent's network of alliances, posing as liberators of the peoples subjected to Athens. Then, since Athens refuses to commit itself to the main theatre, Attica, they seek confrontation in an external theatre: Sicily. The opening of a second front allows the dispersal of the enemy's forces, another great principle of Liddell Hart.

The failure of operations in Sicily resulted in the loss of Athens. This defeat is one of conservatism and maladjustment. The inability to curb the will to expand the empire results in a dispersal of forces. The refusal to build up regular light troops deprives Athens of the operational and tactical mobility essential to its strategy, while Sparta has managed to equip itself with contingents of horsemen and archers.

The work of Thucydides was much studied in Byzantium, but it remained unknown in the West for a long time. The first Latin translation dates from 1513. When from the 16th ^to the 18th ^century, military theorists turned to Antiquity, it is especially in the History of Polybius and not in that ofThucydides that they looked for models and principles, partly because the Athenian neglected the tactical aspects. However, a highly revealing fact is that there is one European country where, from the seventeenth century, Thucydides is ^considered an essential author: England.