The Trojan war, caused by the most beautiful woman in the world, Helen of Troy!
She was, we are told in the myths, the wife of Menelaus, the king of Sparta, but spirited away (either willingly or not) by Paris of Troy, after Aphrodite, the goddess of love, promised him the most beautiful woman in the world as a reward for picking her among all goddesses as the most beautiful. All the Greeks then attacked Troy to get Helen back. Hence the Trojan wars, the trap with the wooden horse and everything else.
Myths are such fun. I've just read two books on the Helen of Troy. She may have been a fertility goddess, she may not have existed at all but she certainly had an impact on Western literature and art. The face that launched a thousand ships!
I've had a theory about Helen's role as the initiator (whether passive or active) of the Trojan wars (whether real or mythical). I consulted the two books on her but found no real mention of my theory, though I'm quite sure that someone else has proposed it before me. Most things have been.
Still, it's a fun theory and goes like this: The Greeks went after Paris and Helen not because she was the most beautiful woman in Greece but because the rule over Sparta belonged to the man who was Helen's husband. By abducting Helen Paris also abducted the kingdom of Sparta. If he could present himself as the husband of Helen he would also have the rule of Sparta.
This is a much more likely theory than the one about Helen's beauty. It is also supported by other parts of the myth, including the myth about Helen's birth.
She is supposed to have been the daughter of Zeus, the boss of the Greek gods, and Leda, the wife of Tyndareus, the king of Sparta. Zeus, in the shape of a swan, either seduced or raped Leda who then laid one or more eggs. Out of the egg(s) hatched Helen, and according to some stories also Clytemnestra and the twins Castor and Pollux.
None of these mythical children are the children of the king of Sparta. But they are the children of the queen of Sparta, Leda. And note that some myths suggest that Helen had full brothers, too.
So why did Menelaus, Helen's husband, become the king of Sparta? Why not Castor or Pollux? Why did the crown of Sparta go to an outsider, the husband of one of its princesses?
The most likely explanation is that the Spartans used the female line to determine inheritance of the crown. A man had to marry the correct princess to become the king of Sparta. That Tyndareus is not Helen's biological father in the myth does not matter if the right to rule was based on Leda being the queen of Sparta. It would then be the husband of her (oldest?) daughter who would be the next ruler.
That the myths assign Zeus as Helen's real father complicates matters, of course. But even if a divine father is preferable to a human one, surely Castor or Pollux would have qualified before Menelaus?
Many aspects of the myth support the idea that the inheritance of city-kingdoms in Greece was once linked to the female line. Even the early abduction of Helen by Theseus fits into the same pattern, and so does the whole idea that a large number of suitors would turn up to compete for Helen's hand:
When it was time for Helen to marry, many kings and princes from around the world came to seek her hand, bringing rich gifts with them, or sent emissaries to do so on their behalf. During the contest, Castor and Pollux had a prominent role in dealing with the suitors, although the final decision was in the hands of Tyndareus. Menelaus, her future husband, did not attend but sent his brother, Agamemnon, to represent him.This looks comprehensible if the kingdom of Sparta went with Helen. But once that link faded from general consciousness something like Helen's beauty had to take its place, to explain both the large number of suitors and the Trojan war.
Old European fairy tales may reflect a similar early inheritance custom. Just think how very common it is in those tales for the hero to get the princess and half the kingdom. If she was an only child, why not the whole kingdom? Perhaps the hero and the princess were once expected to rule together?