[C]oncessions to adversaries only end in self reproach, and the more strictly they are avoided the greater will be the chance of security. - Thucydides

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)

For their (Odrysian Empire) custom was the opposite of that which prevailed in the Persian kingdom; they were more ready to receive than to give; and he who asked and was refused was not so much discredited as he who refused when he was asked.The same custom prevailed among the other Thracians in a less degree, but among the Odrysae, who were richer, more extensively; nothing could be done without presents.

By these means the kingdom became very powerful, and in revenue and general prosperity exceeded all the nations of Europe which lie between the Ionian Sea and the Euxine; in the size and strength of their army being second only, though far inferior, to the Scythians.

(Book 2 Chapter 97.4-5)

At any point where they saw the Corcyraeans distressed, the Athenians appeared and kept the enemy in check; but the generals, who were afraid of disobeying their instructions, would not begin the attack themselves.

[…] At first they had abstained from actual collision, but when the Corcyraeans fled outright and the Corinthians pressed them hard, then every man fell to work; all distinctions were forgotten; — the time had arrived when Corinthian and Athenian were driven to attack one another.

(Book 1 Chapter 49.4, 49.7)