But his (Pericles’) successors were more on an equality with one another, and, each one struggling to be first himself, they were ready to sacrifice … - Thucydides

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But his (Pericles’) successors were more on an equality with one another, and, each one struggling to be first himself, they were ready to sacrifice the whole conduct of affairs to the whims of the people.

Such weakness in a great and imperial city led to many errors, of which the greatest was the Sicilian expedition; not that the Athenians miscalculated their enemy's power, but they themselves, instead of consulting for the interests of the expedition which they had sent out, were occupied in intriguing against one another for the leadership of the democracy, and not only hampered the operations of the army, but became embroiled, for the first time, at home.

And yet after they had lost in the Sicilian expedition the greater part of their fleet and army, and were now distracted by revolution, still they held out three years not only against their former enemies, but against the Sicilians who had combined with them, and against most of their own allies who had risen in revolt. Even when Cyrus the son of the King joined in the war and supplied the Peloponnesian fleet with money, they continued to resist, and were at last overthrown, not by their enemies, but by themselves and their own internal dissensions.

(Book 2 Chapter 65.10-12)