Mount Olympus a decadent wedding fest was held and all gods were invited. Eris the goddess of discord, wasn't invited and sought revenge.
Eris appeared at the party and rolled a golden apple amongst the guest. On the apple was written "To the fairest"
Hera, Athena and Aphrodite all claimed the prize. Zeus asked Hermes to appoint an impartial judge to settle things.
Hermes chose Paris, who was the son of Priam, the king of Troy. Each goddess tried to impress Paris with their own special gift. Paris chose Aphrodite, who promised him the most beautiful woman in the world as his wife.
The most beautiful woman in the world was Helen, but the problem was that Helen was the wife of King Greek Menelaus. Paris visits the palace of Menelaus at Sparta, seduces the queen, and abducts her back to Troy
Long story short, the united powers of the Greeks avenges the wounded honor of Menelaus and retrieve Helen. The Greeks traveled to Troy and for nearly ten years, the Greeks unsuccessfully besieged Troy in the Trojan War.