Poseidon was the brother of Zeus, god of the sky, and Hades, god of the underworld. When the world was divided between them, Poseidon became god of the sea.
He competed with Athena to be patron of Athens. Whereas Poseidon offered the city a spring of water, Athena offered the olive tree. The Athenians chose the olive for its wood, oil and food, but Poseidon was still worshipped. His temple at Cape Sounion is a sign to our sailors that they are nearing home.
Our neighbours (and competitors) in Corinth chose Poseidon as their patron, and began games in his honour on the Isthmus, where an international festival in honour of Poseidon was founded to compete with Olympia, Delphi and Nemea. In Athens we hold our competitions as part of the Great Panathenaic Festival.