Brewer's: Helen of One's Troy
(The). The ambition of one's life; the subject for which we would live and die. The allusion, of course, is to that Helen who eloped with Paris, and thus brought about the siege and destruction of Troy.
For which men all the life they here enjoy Still fight, as for the Helens of their Troy.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894