In William Butler Yeats' poem "Leda and the Swan," discuss how according to William Butler Yeats' view, Leda's rape marks a turning point in history?
Well, that is not (in my opinion) the primary message of the poem, which focuses on the experience of the subject of a classic myth, but that is part of the poem.
Leda was the mother of Helen (among others). Zeus raped Leda for her beauty and his lust alone, but in doing so he fathered children. Helen was the prize given to Paris for choosing Aphrodite as the fairest of the goddesses in a great mythic beauty contest. Since she was already married, when Paris took her to Troy, this started the war. The war was foundational in Greek myth and history. Zeus starts this all in motion with his casual lust, and Leda, in the pain of her rape, must glimpse this as well.