1) The goddess Thetis was prophesied to bear a son greater than his father, so Zeus arranged for her to marry the mortal Peleus. They had a son, Achilles, who was fated to die at Troy.
2) Paris, prince of Troy, was chosen to judge a beauty contest between Hera, Athena, and Aphrodite. He chose Aphrodite, who rewarded him with Helen, wife of Menelaus. Paris took Helen to Troy, starting the Trojan War.
3) Agamemnon led the Greeks to retrieve Helen. However, to appease Artemis and get winds to sail, he sacrificed his daughter Iphigenia