Poseidon, the god of the sea, is angry with Odysseus for several reasons, primarily due to an incident involving Odysseus’ actions against his son, the Cyclops Polyphemus. When Odysseus and his men find themselves trapped in the cave of Polyphemus, Odysseus devises a clever plan to escape. He blinds the Cyclops after getting him drunk and telling him his name is ‘Nobody.’ Once Polyphemus is blinded, he calls for help, saying ‘Nobody’ is attacking him, which causes the other Cyclopes to ignore his pleas.
After escaping, Odysseus reveals his true identity to Polyphemus, boasting about his cleverness. This act of hubris angers Poseidon, as Polyphemus is one of his children. Poseidon, seeking revenge for the blinding of his son, vows to punish Odysseus, making his journey home to Ithaca a long and treacherous one. Throughout the epic, Poseidon creates various obstacles for Odysseus, such as storms and shipwrecks, demonstrating his anger and desire for retribution.