Why Plutarch reads him
Plutarch reads Nicias as the study of caution carried to a fault — a wealthy, honest, religiously scrupulous Athenian whose prudence served his city well in moderation and destroyed it in excess. The platform reads him, with Plutarch, as the mirror-image of Alcibiades: where Alcibiades' ruin came of too much daring, Nicias' came of too little, and the Sicilian Expedition that both men shaped became the grave of Athenian power partly through the recklessness of the one and the hesitation of the other.
Character: prudence and its shadow
Plutarch's portrait is sympathetic and severe at once. Nicias was a good man — careful, conscientious, generous with his wealth, anxious to do right and to be seen to do right — and his caution gave Athens the breathing space of the peace that bears his name. But the same temperament shaded into timidity, indecision, and an excessive deference to omens and priests. The platform reads this under character and power: Nicias' virtues and his fatal flaw were the same disposition seen in two lights, and command exposed the flaw that private life had concealed.
The political significance
Nicias opposed the Sicilian Expedition, was appointed against his will to help lead it, and then — when retreat might still have saved the army — delayed the withdrawal because an eclipse of the moon frightened him into waiting, on his seers' advice, for a further cycle of days. The platform reads this as one of antiquity's most consequential failures of judgement: the hesitation born of his scrupulous piety doomed the expedition, and with it the flower of Athenian manpower and Athens' chance in the war. He died in the catastrophe, executed after the surrender. Prudence, untimely, had become the agent of disaster.
The lesson Plutarch draws
Plutarch pairs Nicias with Crassus — two wealthy men whose foreign expeditions ended in annihilation, the one through over-caution, the other through over-reach. The platform reads the lesson under leadership and character: that the virtues of a leader are situational, that caution and daring are each fatal out of season, and that judgement — the capacity to know which the moment requires — is the governing virtue that Nicias, for all his goodness, lacked at the decisive hour.