Why should I blame her that she filled my days
With misery, or that she would of late
Have taught to ignorant men most violent ways,
Or hurled the little streets upon the great,
Had they but courage equal to desire?
What could have made her peaceful with a mind
That nobleness made simple as a fire,
With beauty like a tightened bow, a kind
That is not natural in an age like this,
Being high and solitary and most stern?
Why, what could she have done, being what she is?
Was there another Troy for her to burn?
It is necessary to know that this poem is about Maud Gonne, with whom Yeats had a turbulent relationship for many years. She was his muse, and manifests in his poetry under a lot of different guises, here appearing as a Helen of Troy figure. The poem is also set against the background of Irish nationalism, particularly at a time when Irish oppression under the British crown was harsh and strong. I am not an expert in this subject, and will try my best to not make sweeping conclusions. I'd encourage you, reader, to look up both Maud Gonne and Yeats' politics at a later date.
In the poem itself, Yeats asks rhetorical questions, trying to reconcile his hurt with the woman who caused them. Why should he blame her for his own hurt, when it is in her nature? Ignorant masses (violent Irish nationalists) follow her due to her beauty and political sentiment, and she has through this "taught to ignorant men most violent ways." He ultimately concludes, "Why, what could she have done, being what she is?" It's her nature, and because there is no Troy for this Helen to see burn, it must be Ireland.
If you can help elucidate this for me, reader, please leave a comment.