After cimon ostracism in the spring of 461
b.
c.
e., athens made an alliance with the city-state of sparta's traditional enemy

Athens made an alliance with the city-state of ARGOS, Sparta's traditional enemy.

Back then, Athens and Sparta were known for two opposing things. Athens was known for their intellectual prowess while Sparta was known for their Military prowess.

Athens see Sparta as a threat, so they formed an alliance with Argos which was considered as the only state that was capable to rival Sparta in term of their military prowess