Someone sent the Gods a golden apple with the message: "For the most beautiful of the Goddesses."
Athena, Aphrodite, and Hera fought over the apple, and decided to have Zeus decide who's the most beautiful.
Zeus couldn't decide. He wanted to choose Aphrodite, but he didn't want to anger his wife, Hera. He also couldn't choose his wife, because that would be favoritism.
So he decided to have a human choose: Paris, a prince of Troy.
The Goddesses each tried to bribe Paris.
Choose me, and you'll never lose a fight, said Athena.
Choose me, and you'll never want for a better household, said Hera.
Choose me, and the most beautiful woman on Earth will fall for you, said Aphrodite.
Paris chose the woman, who happened to be the wife of Menelaus (a king of Sparta), and took her to Troy.
Thus the Trojan war started.
By the way: Athena helped defeat Troy.
Should've picked the Goddess of War, right?