The brutal rape of Leda by Zeus, who disguised himself as a swan to catch her unawares, led to Leda being impregnated by this most powerful of gods. As a result of this rape, Leda gave birth to Helen of Troy. Male competition over Helen, the most beautiful woman in the world, according to Greek legend, resulted in the long and bloody Trojan war.
Yeats, therefore, shows that individual violence and the collective, political violence of war are connected. They are both rooted in the same desire for domination, the same lust to possess, and the same lack of empathy toward the vulnerable. The poem describes Leda as "helpless" and "mastered by the brute blood of the air." Afterwards, having gotten what he wanted, Zeus is "indifferent." In the end, this callous act leads to
The broken wall, the burning roof and tower
And Agamemnon dead
The above are all references to the destruction of the Trojan War. Violence begets violence.
Like many of his time period, Yeats was deeply shocked by World War I, a conflict which unleashed unprecedented levels of slaughter and ripped away the illusion that the world was progressing to become more civilized. Writing not long after the war (this poem was composed in 1923), Yeats was concerned with cycles—or, as he called them, "gyres"—of history. He feared that World War I had ushered in a period of violence similar to the violence that Zeus's rape began.