Helen of Troy was not a Trojan she was Greek and living very comfortably on the Peloponessus with her husband Menelaus when Paris, a son of Priam, king of Troy, came to visit and stole her away. How he did this is unclear, but some say physical force and others say persuasion, which to the Greeks of the day is another type of force. The assorted kings and heroes of Greece were obligated to get Helen back from their Trojan neighbors as a condition of them getting to pursue her hand. Now you ask, weren't all of these loons with funny names Greek? Well, no. The Trojans were across the Hellespont, the body of water that splits Greece and Asia Minor. The Greeks besieged Troy for ten years, finally taking it while lead by the son of Achilles Neoptolymus, a real killer if you've ever seen one. The epics The Illiad and The Aeneid both cover Troy, though the Aeneid only starts there. Helen comes up again in The Odyssey, where she and Menelaus have an interesting exchange.
Here's a note. Helen is the daughter of Leda and sister to Clytaemnestra. Cly murdered her husband Agamemnon, Menelaus' brother, in the play Agamemnon by Aeschylus. One motive is that the Atreids, Agamemnon and Menelaus are symbolized by eagles, but Cly and Helen are the daughters of Zeus in swan form. Swans and eagles are archenemies in Greek myth, making for a great pair of marriages.