The Arsenal

The Athenian trireme was the decisive weapon of the war's naval phases. Crewed by 170 thetes (citizen rowers) and 30 marines and officers, the Athenian trireme was the most agile warship of its era — capable of the 'diekplous' (breaking through enemy lines) and 'periplous' (encircling) manoeuvres that defined Athenian naval superiority. Athens maintained at its height over 300 triremes; the Sicilian Expedition deployed 134. The loss of most of Athens's experienced rowers to the Plague of Athens fatally degraded the quality that made Athenian triremes superior to rivals who had equivalent ships but less trained crews.

The Spartan hoplite's panoply — Corinthian bronze helmet, bronze cuirass or linen linothorax, bronze greaves, large aspis shield, and eight-foot dory spear — was the standard armament of the finest infantry in the Greek world. Spartan hoplites trained from childhood through the agoge and fought with a discipline and cohesion unmatched by citizen militias. At Mantinea (418 BC), the largest land battle of the war, Spartan tactical precision crushed a coalition that outnumbered them on paper. The hoplite panoply changed little across the war's twenty-seven years; it was the training and willingness to use it that divided Spartan infantry from all others.

The bronze-clad ram fitted to the bow of a trireme at the waterline was the primary naval weapon of the Peloponnesian War. A trireme at full speed (7–8 knots) could punch a hole in an enemy ship's hull large enough to sink it in minutes; even a glancing blow could shear off oars and disable a vessel. Athenian rowers practiced specific manoeuvres designed to maximise the ram's effectiveness: the diekplous involved rowing through the enemy line and ramming ships from behind; the periplous meant encircling and ramming from the flank. As the war progressed and Sparta built fleets, both sides increasingly reinforced their bows and switched to ramming-and-boarding hybrid tactics.

The Peloponnesian War saw significant development in Greek siege warfare, which had previously consisted largely of blockade and direct assault. The Spartan siege of Plataea (429–427 BC) involved elaborate circumvallation walls surrounding the city, attempts to set the wooden palisade alight using sulfur and pitch, and the famous escape of most of the garrison by ladder over the siege wall. Athens's siege of Syracuse involved extensive earthworks and counter-walls. These operations developed the engineering infrastructure — ramps, mounds, towers, and tunnelling — that Alexander the Great would use to make systematic siege warfare a decisive strategic tool.

The Peloponnesian War saw the first systematic use of fire as a military weapon in Greek warfare. At the siege of Plataea (427 BC), the Spartans attempted to set the city ablaze using a large fire created from wood, sulfur, and pitch — one of the earliest recorded uses of sulfur as a chemical incendiary in warfare. At the siege of Delium (424 BC), the Boeotians used a primitive flame weapon — a hollowed-out wooden beam connected to bellows, projecting burning charcoal and pitch — to set fire to Athenian fortifications. This device is sometimes described as the ancestor of the flamethrower and is one of the earliest recorded directed-flame weapons.

Peltasts — light-armed Thracian mercenary infantry carrying a bundle of javelins and a light wicker shield (pelte) but no heavy armour — played an increasingly important role as the war progressed. At Sphacteria (425 BC), peltasts and archers sent over from Athens were central to the defeat of Spartan hoplites who could not come to grips with enemies who skirmished and withdrew. The general Demosthenes used Aetolian and Acarnanian light troops extensively. The Peloponnesian War demonstrated that the heavy hoplite phalanx had vulnerabilities to mobile light infantry in terrain where the phalanx could not maintain cohesion — a lesson that Iphicrates formalised in the 390s BC.

Cavalry played a more significant role in the Peloponnesian War than in the Persian Wars, particularly in the campaigns of northern Greece and Sicily. Thessalian cavalry — the finest in Greece — served both sides at different points. Boeotian cavalry (heavily armed) were decisive at Delium (424 BC). In Syracuse, the Syracusan cavalry was a constant torment to the Athenian army, cutting off foragers, destroying supply columns, and finally driving the retreating army into the Assinarus River ambush. Athens had always had inferior cavalry — a weakness of its geography and its democratic system's emphasis on naval power — that proved fatal in the open terrain of Sicily.

Athens's Long Walls — connecting the city to the port of Piraeus across four miles of flat ground — were the physical expression of Pericles' naval strategy. As long as the walls stood, Athens could not be starved into submission by land; the fleet controlled the sea lanes and brought grain from the Black Sea regardless of what happened on land. The walls were Athens's most important military asset, more important than any single fleet or army. When Lysander's victory at Aegospotami severed the sea route, the walls became useless, and their demolition — to the sound of flutes, with the Greek world watching — marked the formal end of Athenian power.

As both sides' naval capabilities converged after the Sicilian disaster, sea battles increasingly involved boarding actions rather than pure ramming manoeuvres. Marines (epibatai) carried short swords, javelins, body armour, and grappling hooks for pulling enemy ships alongside. Spartan and Corinthian ships — built heavier than Athenian triremes — were designed specifically for boarding combat, with reinforced bows for head-on ramming. At Arginusae (406 BC), the largest naval battle of the war, both sides used a combination of ramming, boarding, and missile fire in a melee that lasted much of the day. The evolution toward boarding-focused naval combat anticipated the Roman corvus that would revolutionise naval warfare in the Punic Wars.

The xiphos was the hoplite's sidearm — a double-edged short sword of about 50–60 cm, carried for use when the spear broke or combat became too close for its length. Sparta was famous for training its warriors to fight effectively even when their spear was gone; the xiphos training was part of the agoge. At Sphacteria, when the Athenians and allies pressed the Spartan hoplites from multiple sides in terrain that negated phalanx formation, the Spartans' ability to fight individually with the xiphos — rather than breaking and running as most citizen militias would have — was what allowed them to make an orderly final stand rather than a rout.