Once upon a time the sea goddess Thetis was forced to marry the mortal King Peleus.

They had the wedding of the century, and all the Greek deities were invited except Eris, the goddess of strife. Insulted, Eris crashed the wedding and tossed onto the dance floor a golden apple inscribed "For the Most Beautiful." Three goddesses immediately demanded the apple-Hera (wife of Zeus), Athena (goddess of warfare and wisdom), and Aphrodite (goddess of love and beauty). None of the gods wanted to arbitrate that dispute (being immortal, they would literally never hear the end of it), so they asked the Trojan prince Paris to serve as the judge. All three goddesses tried to bribe him —Hera with kingship over Asia, Athena with outstanding wisdom, and Aphrodite with the most beautiful woman in the world as his bedmate. Paris chose Aphrodite, and got in exchange Helen of Sparta. Problem was, Helen was already married to Menelaos, king of Sparta and brother of Agamemnon, king of Mycenae.

So when Paris seduced Helen and brought her back to Troy, it started an international incident. For ten years the Greeks tried using diplomacy to get Helen returned. Then they went to war, which lasted another ten years.