The actual trigger was Athens' ruinous trading ban on its neighbour Megara, which was a member of the Peloponnesian League headed by Sparta which was opposing Athenian expansionism. The League asked Athens to rescind the ban, Athens refused and war broke out.
As Greeks, their cultures were very similar. Their alliance was for self interest - Sparta aided Athens, and Athens reciprocated.
Sparta
Athens and Sparta had a pantheon of gods headed by Zeus.
Athens and Sparta had a pantheon of gods headed by Zeus.
Athens had not been a kingdom for half a millennium. Athens government was democratic, headed by ten Archons (magistrates). The army and navy were commanded by ten generals headed by the War Archon.
The address of the National Alliance Of Preservation Commissions is: Po Box 1605, Athens, GA 30603-1605
It allowed Athens to convert the anti-Persian alliance which it headed against the Persian Empire, after peace was arranged, into an empire of its own, which created a standoff withe the Peloponnesian League headed by Sparta. This standoff culminated in the destructive 27-year Peloponnesian War between the two blocs.
It was between Athens and its empire and the Peloponnesian League headed by Sparta.
Athens.
The Delian League
Many city states formed an alliance against Athens because they felt the Athenian's treated them as mere subjects. Athens is the capital of Greece.