It shifted over time as did all the Greek city-states. Originally friendly with Sparta, this fell apart 50 years later. Athens also turned the anti-Persian Delian League into an empire of its own, living off it to finance its Golden Age. It fought Thebes, then allied to it, and sided with many other city-states when convenient.
The Delian League
The Peloponnesian War was between alliances of Greek city-states - Athens and its empire, and the Peloponnesian League led by Sparta.
At one point it was Athens but they turned on Sparta.
No, they welcomed foreigners (under clear conditions) which helped the growth of culture. The also kept a close relationship to other Greek city-states, in alliances and establishing an empire.
There are many different kinds of alliances, and many different reasons for entering into alliances, but generally speaking, people form alliances because an allied group of people can often accomplish things that a single individual cannot.
Hernán Cortés arranged alliances with the enemies of the Aztecs.
peloponnesian league
The Delian League, which morphed into an empire of Athens.
They were both superpowers in Ancient Greece, and were the leaders of their alliances - the Peloponnessian league and the Athenian Empire.
Athens. However, it is worth noting that during the Era of Ancient Greece, the city-states were not united in anything other than local alliances. As a result, Athens was only the capital of the Attica region.
The Peloponnesian War was between alliances of Greek city-states - Athens and its empire, and the Peloponnesian League led by Sparta.
At one point it was Athens but they turned on Sparta.
They united 180 Greek city-states into several alliances, the first led by Sparta, the next led by Athens.
~Alliances~Alliances~Alliances~Alliances~
Eurymedon 468 BCE and Cyprus 450 BCE. There were earlier battles between the Persian fleet and alliances of Greek city-states including Athens.
It was not limited to three. There was Sparts, Thebes, Corinth, Argos, Athens and a couple of dozen others, all shifting alliances as it suited them.
No, they welcomed foreigners (under clear conditions) which helped the growth of culture. The also kept a close relationship to other Greek city-states, in alliances and establishing an empire.
The Greek city-states were never united, they formed shifting alliances. The Peloponnesian War was between two groups, Athens and it's empire and the Peloponnesian League led by Sparta. The League won the war, not Sparta. When Athens was defeated, it was stripped of its empire, and became a second rate power. Without the threat of Athens, the city-states began to form other regional alliances which suited their own individual interests and so some of the allies against Athens realigned themselves, with eventually Thebes becoming dominant against a Sparta short on manpower after years of war losses.