Yeats's poem "Leda and the Swan" is based on a story from Greek mythology in which the God Zeus takes on the form of a swan and, in that form, rapes a girl named Leda. There is a child born of this rape, Helen of Troy. Years later, Helen is abducted by a prince called Paris, and this abduction leads to the Trojan War. A thousand ships are sent to rescue Helen, who, in the stories, is greatly admired and renowned for her beauty. Indeed, Christopher Marlowe once said of Helen that hers was "the face that launched a thousand ships."
In Yeats's poem "Leda and the Swan," the speaker imagines the rape and its aftermath. He describes Leda as "the staggering girl," and in the second stanza he asks, "How can those terrified fingers push / The feathered glory from her loosening thighs?" This rhetorical question emphasizes how powerless and frightened Leda must have been during the rape.
In the second half of the poem the speaker considers the consequences of the rape. He says that "a shudder in the loins engenders there / The broken wall, the burning roof and tower." The meaning here is that when Zeus had sex with Leda, it "engener[ed]', or started in that moment, the Trojan War and the invasion of Troy. The "broken wall" alludes at once to the breaking of Leda's hymen during the rape and also to the breaking of the wall around Troy.
In the closing lines of the poem, the speaker wonders if Leda knew that her attacker was Zeus, and if she knew from that moment what the consequences of the rape would be. He asks whether Leda "put on his knowledge with his power, / Before the indifferent beak could let her drop?" In other words, did Leda receive not only the terrible physical power of Zeus in this moment but also the terrible knowledge of the destruction that would eventually ensue?