In the poem “Leda and the Swan” by W. B. Yeats, the poet seeks to demonstrate how one event can have dramatic repercussions throughout human history.
The highly unpleasant event depicted in the poem is the rape of the Spartan queen Leda by Zeus, the king of the Greek gods, disguised as a swan. Although this is a mythological event involving two fictitious characters, in ancient Greece it was widely believed that Zeus's violation of Leda led eventually to the Golden Age of Greek culture, one of the cornerstones of Western civilization.
It's notable in this regard that Yeats passes no judgment on the morality of Leda's rape; he simply treats it as a significant historical event with profound repercussions that continue to reverberate down the centuries.
Yeats believed that human history was divided up into gyres, historical eras depicted in pictorial form by large cones in which gyres intersect at various angles. The end of one gyre and its replacement by another would take place roughly every two thousand years or so and would be precipitated by an important event. One such event was the birth of Christ; another was the rape of Leda by Zeus in the guise of a swan. Though different in so many ways, both of these events were, in Yeats's eyes, foundation stones in the building of Western civilization.