It all started when Eris, the goddess of discord, realized that she had not been invited to the marriage of the King Peleus, with the sea Nymph Thetis; a marriage that took place in the Mountain of Pelion in Thessaly (Central Greece).
Eris was so furious about her exclusion that she decided to go to the wedding and throw a golden apple, the fruit of temptation, to the banquet table. The apple was supposed to go out to the "Calliste"- the fairest one.
Three goddesses claimed the beautiful golden apple: Hera, the goddess of Marriage, Athena, the goddess of Wisdom and Aphrodite, the beautiful goddess of Love. Zeus had a hard time to decide, since all three were dear to him, so he handled the judgement to the most handsome youth, the Trojan Prince Paris.
Hera and Athena did their best to bribe Paris with power and glory; Aphrodite, on the other hand, seduced him with love, offering him the most beautiful female mortal in the world, Helen of Sparta. Paris opted for Aphrodite and so Helen was taken from Sparta to become Helen of Troy.