Helen of Troy was originally the Greek Queen of Sparta who was married to King Menelaus.
According to one myth, Helen was the child of Leda and Zeus. Leda bore Helen and Polydeuces from one egg and the offspring of her husband Tyndareus, the King of Sparta, from the another egg. The other set of twins were named Castor and Clytemnestra. In other versions, Helen is a daughter of Nemesis, the goddess who personified the disaster that awaited those suffering from the pride of hubris.
Helen was married to Menelaus. Since both Polydeuces and Castor (the only male offspring of Leda) had died already by the time Tyndareus passed away, Menelaus became King of Sparta.
Some myths say that Helen was bewitched by Aphrodite into loving Prince Paris of Troy (see Apple of Discord)
see also: Helen of Troy. Others say she fell in love with him and willingly left behind Sparta, Menelaus and their nine year old daughter Hermione. In Euripides' play Helen, Hermes creates a likeness of Helen out of the clouds and Helen spends the entire war hiding in Egypt.
After the war, Helen returned to Menelaus and Sparta. Upon his death, she was exiled by their son, Megapenthes. Whilst Helen was undoubtedly a beautiful woman, if this myth is indeed based in fact, then there is a far more cynical reason for the fight over her. Menelaus' claim to the Spartan throne was based on his marriage to the daughter of the previous King (the matrilineal inheritance system).