Why does Odysseus have to be away from home for 20 years?

Odysseus had to spend ten years fighting in the Trojan War and another ten years trying to get home because of the wrath of Poseidon, god of the sea. At the end of the war, while the Greek armies are plundering and burning Troy, Odysseus angers Poseidon by blinding his mortal son, the giant, one-eyed Cyclops Polyphemus.