The poem ends on this simple and pithy question, which is like the punch line after an elaborate setup.
Considering that the title of the poem is "No Second Troy," we were wondering when the ancient city of Troy was going to make its big entrance. Yeats creates suspense by delaying this until the last line.
The mention of Troy ties together the little hints about how Maud Gonne is like a character from an ancient Greek epic. In this line, he compares her directly to the famous Helen of Troy.
Helen of Troy was considered the most beautiful woman in ancient Greece, maybe even the most beautiful woman of all time.
Helen was also seen as being responsible for the Trojan War when she ran off with the Trojan Prince Paris, abandoning her husband King Menelaus, who became very, very angry.
Helen's was "the face that launched a thousand ships." She was viewed from then on, rightly or wrongly, as a beautiful troublemaker.
At the end of the war, Troy is burned to the ground and lots of people die.
The speaker is saying that, if there had been another Troy for Maud to burn, she would have burned it. But since there wasn't, she had to go around making trouble for lovesick poets and stirring up violent protests against the British.
He compares the magnitude of the problems Gonne created to the destruction of an entire city. Sucker-punch!
And that, ladies and gentleman, is how to spurn a former lover.