After finding out she wasn't invited, Eris, the goddess of discord was furious. She rolled in an apple that said "To the Fairest." Athena, Aphrodite, and Hera all fought over the apple.
To decide who is really the Fairest, Zeus brought a mortal, Paris to Mount Olympus. He had to choose between the goddesses claims, and he chose Aphrodite's that promised him the most beautiful woman on earth.
Paris visited the palace of Menelaus at Sparta where he was hoping to find Helen. He abducted her and took her back to Troy. The Greeks responded in outrage and the united powers had to avenge Menelaus and retrieve Helen.
It took 10 years for the Greeks to travel to Troy, and they did not get Helen back. Then Odysseus built a large wooden horse. The Greeks hid warriors inside and left it at the gates of Troy. Later, the Trojans took the horse into the city and the warriors bombarded the city. That was the end of the Trojan War.