This battle is against Athena the goddess of war and against Poseidon the god of the sea . And Athena had become the patron of the Athens in order to be the patron of Athens,Athena and Poseidon had a battle .So Athena caused an olive tree to spring up on the citadel of Athens, the Acropolis and since they are able to eat the olives they thought it was useful.And Poiesenden used his trident and by striking it on the ground, a gush of spring water flowed forth.Since the water was salty and the Athens thought Poseidon’s gift was useless ,so this is how she overthrone Poseidon.