The defeat that needs explaining
Athens was the richer, more dynamic, more creative power, and it lost the long war. The platform reads the question why Athens lost as one of the permanent puzzles of strategy, and reads Thucydides' answer as uncomfortable: Athens was not defeated so much as it defeated itself. The city that had the resources, the fleet and the strategy to prevail instead threw its advantages away through a sequence of avoidable choices — and the history of its defeat is a history of self-inflicted wounds.
The abandonment of Periclean strategy
The platform reads the first cause as the abandonment of Pericles' grand strategy. Pericles had given Athens a sound plan: rely on the navy, avoid pitched land battles against the superior Spartan army, outlast the enemy, and — crucially — add nothing to the empire while the war was unwon. The strategy required patience and restraint. After Pericles died of the plague in 429 BCE, the democracy lacked both: it grew aggressive under Cleon, gambled on expansion, and squandered the position of strength Pericles had built. The platform reads the defeat as beginning here, in the loss of strategic discipline.
Sicily and faction
The platform reads the decisive blow as the Sicilian Expedition — the vast overseas gamble, against an enemy that threatened nothing, that ended in the annihilation of Athens' fleet and the flower of its manpower at Syracuse in 413 BCE. And it reads the war's later years as a study in civil war and stasis: oligarchic coups, the recall and re-exile of Alcibiades, the execution of victorious generals by the assembly after Arginusae, a democracy turning on itself even as it fought for survival. The platform reads faction as the internal counterpart to the external defeat.
The sea, and Persian gold
The platform reads the final cause as the one thing Athens could not control: the entry of Persian money on Sparta's side. Sparta could never beat Athens at sea with its own resources; it could only do so once Lysander used Persian gold to build a fleet, and destroyed the last Athenian navy at Aegospotami in 405 BCE. The platform reads the conjunction as the lesson: Athens defeated itself strategically and politically, and Persia supplied the means for Sparta to finish it. The richer power lost because it lacked the restraint to keep what its strategy had won — the permanent caution of strategic failure.