Claude Debussy completed only one opera, Pelléas & Mélisande, but actually worked on five. He also wrote for the theatre in other ways, with incidental music for plays and music for ballet.
However he was known to be not so fond of the world of theatre and not terribly adept at negotiating the politics of it.
In the 1890's opera world there was the influence of Massenet, of verismo and especially of Richard Wagner.
Debussy respected these but in his own work he wanted to go in another direction, more influenced by the Symbolist movement in poetry and drama.
Music Details:
DEBUSSY: Rodrigue et Chimène
DEBUSSY: Fall of the House of Usher
DEBUSSY: Pélleas et Mélisande