What is one of the main (but not obvious) themes of the poem "Leda And the Swan" by William Butler Yeats?
I don't mean mood or topic, or anything like that, I mean a theme that's like a lesson to be learned.
"Leda and the Swan" is based on the Greek myth in which Zeus, in the guise of a swan, rapes Leda, who then gives birth to Helen of Troy. One of Zeus's unfortunate habits was an attraction to mortals, and he often appeared to them as an animal which then raped them As is not uncommon with acts performed based on raging hormones (lust), there are often unintended consequences of these acts, and in the case of Leda's rape, the consequences are world shaking.
As I noted, Leda gives birth to Helen of Troy, who eventually marries Menelaus, King of Sparta, and after a visit by Hector and Paris of Troy, she either voluntarily goes with, or is simply taken by, Paris. Menelaus, without sufficient troops to fight a successful war against Troy, seeks and receives Agamemnon's help to assemble a coalition of Greeks to fight Troy and return Helen to Menelaus.
Before Agamemnon commits himself to the war, however, he sacrifices his daughter, Iphigenia, to insure victory, a cold-blooded murder that earns him the hatred of his wife Clytemnestra. When Agamemnon returns from the Trojan War victorious, his wife and her lover kill him--Clytemnestra has been planning his murder since he sacrificed their daughter.
The Trojan War itself killed tens of thousands of Greek and Trojan troops, most of the Trojan royal family, including Hector's child, essentially wiping away a dynasty that had lasted for hundreds of years and stopped Greek expansion eastward. Among the Greek dead, of course, were warriors like Achilles and Ajax. An entire decade was consumed by the war, and even the victors--Odysseus and Aeneas, for example--continued to pay for the war long after its conclusion.
When Zeus gave in to his lust for Leda, it is unlikely that he thought about the obvious consequence of his actions, the birth of a child, but the unrecognized and unintended consequences essentially shifted the balance of power between Greece and Troy to Greece, and Troy is remembered only as the subject of a poem.