Odysseus originally left Ithaca to participate in the Trojan War, according to SparkNotes. The Greeks attacked the Trojans in order to return Helen of Troy to Menelaus, and as one of Helen's original suitors, Odysseus had sworn an oath to assist Menelaus. While he did try to get out of his obligation by pleading insanity, he eventually honored his promise and sailed off to war.
During the Trojan War, Odysseus proved to be one of the most valuable strategists and leaders on the Greek side of the conflict. He led many victorious raids, and he was the creator of the famed Trojan horse that proved instrumental in allowing the Greeks into the city. While Troy eventually fell, Odysseus' actions during the long siege did not go unnoticed.
When Odysseus killed the son of the Trojan hero Hector, the boy's grandmother prayed to the gods for revenge, and the gods granted her wish. Odysseus was blown off course during his return from the Trojan War, and he spent 10 years traveling from island to island attempting to find his way home. The trials of this journey, along with what happened when he finally returned to his homeland, are chronicled in Homer's epic, "The Odyssey."