"Leda and the Swan" is written in the style of a Petrarchan sonnet. The first eight lines (octave) describe an event and the final six lines (sestet) resolve the action. Yeats never mentions Zeus or Leda in the poem itself so he was counting on the reader to know the story. Zeus raped (or seduced) Leda, a mortal, while he was in the form of a swan. From that, Helen, Clytemnestra, Castor, and Pollux were born. These were all key players and/or literary characters who took part in events surrounding the Trojan War and the transition from the end of early Greek civilization and the birth of a more modern era. The poem focuses on a particular event, Leda's rape and conception, set to the backdrop of a larger historical transition. The question remaining at the end of the poem is just how much Leda is aware of the significance of her part in this larger historical moment. One very significant theme in this poem is a theory of history.
The brutality of the event suggests that even as people have relative free will, they are subject to a kind of cosmic (or mythological) force of history. And the other implication is that, cosmic or man-made, historical shifts can be violent. A historical connection to the poem, although not alluded to in the poem, is the Irish Civil War. Yeats supported the Free State of Ireland but did not like the violence that resulted from this struggle for independence.
An earlier version of the poem was called "Annunciation." This is an obvious reference to the Annunciation in Christianity when Mary was visited by the angel Gabriel who told her she would bear the son of God. The Holy Spirit (in the form of a dove) fulfilled this prophecy. A union between humanity and the divine would be terrifying and would seem so to a mortal more than it would to a God. A clash of civilizations, cultures, or ideas would also be turbulent; historical transitions have often been similarly turbulent.
In the Annunciation and in Leda's story, the offspring bring a new era of history to the world. Yeats believed in cyclical periods of history. He thought that a dramatic change would occur every two thousand years. During such a dramatic change, things would be disconcerting, turbulent, and possibly violent. This poem symbolizes Yeat's theory of history as it might be understood in natural (physical) and spiritual terms.
The poem's octave (first 8 lines) describe Leda's rape/seduction. Yeats does not gloss over the violence of the act. So, he notes that violence occurs at historical transitions but by dramatizing the violence, he condemns it, at least as it manifests because of violence in humanity. He ends the poem with the question of whether Leda (or anyone caught up in a historical moment) realizes the significance of the event:
Being so caught up,
So mastered by the brute blood of the air,
Did she put on his knowledge with his power
Before the indifferent beak could let her drop?