The causes which led to the defections of the allies were of different kinds, the principal being their neglect to pay the tribute or to furnish ship… - Thucydides

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The causes which led to the defections of the allies were of different kinds, the principal being their neglect to pay the tribute or to furnish ships, and, in some cases, failure of military service. For the Athenians were exacting and oppressive, using coercive measures towards men who were neither willing nor accustomed to work hard.

And for various reasons they soon began to prove less agreeable leaders than at first. They no longer fought upon an equality with the rest of the confederates, and they had no difficulty in reducing them when they revolted.

Now the allies brought all this upon themselves; for the majority of them disliked military service and absence from home, and so they agreed to contribute their share of the expense instead of ships. Whereby the Athenian navy was proportionally increased, while they themselves were always untrained and unprepared for war when they revolted.

(Book 1 Chapter 99)