Under Two Flags (novel)

[1] The most famous of her books, it tells the story of an English aristocrat, apparently in disgrace, who disappears and joins a French battalion in Algeria, loosely based on the Foreign Legion.

[2][3] In financial distress because of his own profligacy and the loss of an important horse-race on which he has bet extensively, and falsely accused of forgery, but unable to defend himself against the charge without injuring the "honour" of a lady and also exposing his younger brother (the real culprit), Cecil fakes his own death and exiles himself to Algeria where he joins the Chasseurs d'Afrique, a regiment comprising soldiers from various countries, rather like the French Foreign Legion.

After Cecil's great childhood friend and the friend's beautiful sister show up in Africa, and after a series of melodramatic self-sacrifices by Cecil and by the young girl Cigarette, a "child of the Army" who sacrifices her life saving Cecil from a firing squad, the main conflicts are resolved and the surviving characters return to England to fortune, title, and love.

The book has also served as a basis for a number of stage and film adaptations.

Further suggestions might be found on the article's talk page.