The Iliad begins in the tenth year of the war and does not tell how it started, but it makes the reason clear: Helen. Paris, son of King Priam, took Helen from Sparta to Troy, and Menelaus, her husband, and his brother Agamemnon, leader of the Greeks, came to bring her back with an army. Paris recalls the voyage to her: "after I snatched you from lovely Lacedaemon, I sailed in the ships that cross the sea."
In Book III, the two armies agree to a duel. Paris proposes to fight Menelaus "for Helen and for all the goods," and whoever wins takes the woman home and ends the war. Menelaus prays to Zeus to punish Paris, "so that even the men who are born later will tremble at doing harm to the host who offers them friendship," that is, the offense was against someone who received a guest. The goddess Aphrodite snatches Paris from the fight, and the oath is not fulfilled.
Helen stands between the two sides. In Book III she weaves a cloak on which she embroiders the combats that Trojans and Achaeans suffer "because of her," and the elders of Troy say that it is no wonder men suffer so long over a woman like her, but that she should sail back in the ships. King Priam calls her his daughter and absolves her: "to me you are not to blame; to me the gods are to blame." She herself says she would have preferred to die rather than follow Paris.
Not even the return of Helen is accepted. In Book VII, the Trojan Antenor proposes returning Helen and the goods, and Paris answers that he will return only the goods, not the woman; on the Greek side, Diomedes says that no one should accept either the goods or Helen now. So the war goes on, and the Greeks already speak of the fall of Troy as a settled matter.