Why did Athena and Poseidon have a war?

Athena and Poseidon competed for the patronage of the city of Athens, each offering a gift to the city's inhabitants. Poseidon struck the ground with his trident, creating a saltwater spring, while Athena offered the olive tree, symbolizing peace and prosperity. The citizens chose Athena's gift as more valuable, leading to Poseidon's anger and a rivalry between the two deities. This conflict ultimately established Athena as the city's protector and namesake.