"The broken wall, the burning roof and tower / and Agamemnon dead." Leda's body is broken through penetration, and Troy's wall also becomes broken. Zeus' desire burns, like the roofs and towers of Troy will burn. And men will die, including the great general Agamemnon. Time rushes forward in an instant. Leda's pregnancy resulted in Helen, for whom the Trojan War was waged. Yet the future war is also a kind of synecdoche for the violence done to Leda. The violence of war and the violence of sexuality are intertwined, and become metaphors for one another. The reader is suddenly aware that he or she has been reading an extended metaphor, both for how one sexual act can lead to violence, and also how violence is at the heart of all sexual activity. The poem reaches its climax with the sexual act, which foreshadows the horror to come.

Then, the poem begins to exhibit falling action, or an anticlimax....

The poem ends, not with certainty, but another rhetorical question. It shifts back into the moment of the actual rape from the future war by using the verb "being." Given that Leda was "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?" When one is so close to godlike power, does the human individual gain a sense of the future, of what the sexual act will lead to, Yeats wonders. The end line is so powerful, because of its explosive alliteration of the words "brute blood" and its intense repetition of the word 'so.' Zeus' indifference to his cruelty and his indifference to what his desire spawned is metaphorically demonstrated by the poet's description of his beak.
Works Cited

Yeats. W.B. "Leda and the Swan." Online Literature Library. 11 Nov 2007. http://www.online-literature.com/yeats/865