"No Second Troy" plays out through four rhetorical questions.
First, the speaker wonders "why" he should blame "her" for his unhappiness and for her reckless manipulation of the emotions of Irish commoners to rouse political violence.
Then he asks whether it would even have been possible for "her" to be a "peaceful" person. He thinks her character and beauty have an old-school quality, more like a figure from Greek tragedy than a contemporary woman. She belongs to another age.
She could not have been anything other than what she is. Simple enough.
Last, because there was no "second Troy" for her to destroy, she had to destroy other things – like the speaker's happiness, and the lives of Irish commoners. The first Troy, of course, was destroyed because of a quarrel over Helen, another politically troublesome beauty from another "age": ancient Greece.