Eris and the Apple of Discord
Many people think after reading the Iliad that Helen was the cause of the Trojan War. Well, that's kind of true and kind of not true. The chain of events that started the Trojan War all began with a spiteful goddess and a simple apple.
Zeus was putting together a wedding banquet for Peleus and Thetis. He planned to invite just about everyone. Everyone except Eris. She was known far and wide as a troublemaker so she was left out as she was left out of just about every big reception.
Needless to say, Eris was greatly angered by this. She then thought up a little plot to stir things up at the reception. She created a golden apple and upon it she wrote Kallisti (meaning "To the prettiest one"). On the day of the event, she rolled the apple into the banquet and took off.
This is where the trouble began. Athena, Hera, and Aphrodite each claimed the apple for herself and eventually began to battle over the apple. Why did they fight over it? Well, it did say "to the prettiest one" on it and of course, each of them believed it was her. From what I know of this tale, the fight wasn't a pretty picture.
After watching for a few, Zeus decided to end the quarral and send them to an arbitrator to decide which of them deserves the apple. He sent them to Paris of Troy. Each of the three goddesses tried to out do the other by getting to Paris first and offering a bribe. Athena offered Heroic victories, Hera offered vast wealth, and Aphrodite offered him the most beautiful woman in all the world. Paris of course took Aphrodite's offer, so she took the apple and Paris was given Helen, wife of King Menelaus of Sparta.
This of course started the Trojan War. Menelaus wanted his wife back, Paris would not agree, and boom, the war begins.