The Delian League for Kids
About 200 city-states banded together to form the Delian League. The Delian League was formed to create a treasury. The idea was that all the city-states in the league would pay money into the treasury to create a savings account. That account was to be used, if necessary, to fund a way. War was expense. It took money to make weapons, build ships, and train men.
The Persian Wars had drained the Greeks. The Greeks won, but they wanted to be ready if the Persians ever came back. The Persians had tried three times to conquered the wily Greeks. No one knew if they would try a fourth time, or if someone else might attack the Greek city-states. It was a real threat, and the Delian League was a smart idea.
Athens took the responsibility of holding, guarding, and accounting for the money that poured into the treasury. They didn't ask. They simply started collecting money from all the league members including themselves. Athens charged a small fee, a very small fee, for this service. The other city-states were fine with this. It seemed perfectly reasonable to charge a small fee to offset costs. And someone had to collect the money and store it and guard it. The obvious choice probably was Corinth, but Athens took the lead and no one argued with them, not even Corinth.
Athens only charged 1/60th of the money pouring into the Delian League treasury that they were guarding. Their accounting was quick and accurate. They reported back to the other city-states. They did a good job. The thing was, even 1/60th was a great deal of money. Athens soon became rich.
Sparta complained about it. They thought it was unfair that Athens guarded such a huge amount of money alone. They believed the money should be spread about for safety and fairness. Athens refused. The rest of the city-states refused to get involved. Sparta and Athens were always fussing about something.