It all started with a missing wedding invitation.
When the mortal King Peleus married the nymph Thetis, they held a magnificent banquet to be remembered. The halls were filled with men and women in beautiful gowns, all the most important people from across the country. And of course, to include the bride's side of the family, the guest list also included many of the gods.
Eris, the goddess of Discord, had not been invited to the wedding (and for good reasons!) But Eris was most offended. Jealous, she decided to cause trouble by tossing a golden apple to the guests. "It's for the most beautiful lady here!" she told them, and disappeared with a smirk.
But who was the most beautiful? Three goddesses, Hera, Athena, and Aphrodite, each felt that they deserved the title and started fighting over the apple, much to everyone else's horror. Zeus, the very annoyed king of gods, ordered them to take their argument somewhere else. "Go ask that Trojan prince Paris to judge! I heard he's good at this."
It turned out that all three goddesses were equally beautiful, so they all resorted to bribery. Hera promised Paris endless wealth. Athena promised him glory. Aphrodite promised the love of Helen, the most beautiful mortal girl alive. Always the romantic sort, Paris couldn't resist Aphrodite's offer. The problem was, Helen was already married to one of the kings of Greece.
Paris decided to run away with Helen anyways. Naturally, this caused all of the Greek kings to sail for Troy with a massive army. Troy rallies to defend itself, and thus begins a long, bloody war.
One that you would hopefully end.