What protective group formed by Greeks city states?

ANSWER

There had been three major leagues formed by Greeks city states:

The Peloponnesian League under Sparta's hegemony, formed about the mid of the VI century BC to contrast the Athens's power.

The first Delian-Attica league under Athens's hegemony, formed in 478-477 BC during the last phase of the Persian Wars.

The second Delian-Attica League under Athens's hegemony, formed in 377 BC in opposition to the military alliance between Sparta and the Persian Empire.