In 461 BC, Athens built a wall. A big, beautiful wall. Athens built a great, great wall, actually two great walls, and Athens made the Delian League pay for those walls. Mark my words.
After the Greeks destroyed their Persian invaders in the Greco-Persian Wars, Athens and other city-states formed the Delian League as an alliance to defend against future attacks. Most of the city-states didn’t have military resources to contribute, so instead they paid tribute to Athens and became protectorates.
The Persians stopped being a significant threat after the war, but Athens kept collecting tribute anyway. The Athenians used the tribute money to build their walls, and the Parthenon, and other stuff that had nothing to with the Delian League. Any city-states that tried to secede were attacked and had their land and ships confiscated.
Eventually the city-states got tired of paying for Athens’ dumb monuments so they allied with Sparta and destroyed Athens in the Peloponnesian War. Then they tore down the walls. The End.