Why did Armand St. Just return to France?

In Chapter 6, "An Exquisite of '92," from The Scarlet Pimpernel, Marguerite becomes increasingly dismayed about her brother's return to France. She has so enjoyed his visit to her home in England, especially because of her husband's now distant and cold relationship with her. As she prepares to say goodbye to her only kin, she is troubled by the merry nature of Sir Percy and his band of friends and asks to be able to say farewell in private. At the end of the chapter, as Sir Percy silently admires the loyal bond between his new wife and her brother, he provides readers with the surface reason for Armand's return to France. Sir Percy observes that Marguerite's "love for her brother, Armand St. Just, was deep and touching in the extreme. He had just spent a few weeks with her in her English home, and was going back to serve his country, at the moment when death was the usual reward for the most enduring devotion."
Even though Armand's cause—at this point in the novel—is at odds with Sir Percy's as the Scarlet Pimpernel, Percy can still give credit to the young man for being willing to risk and/or sacrifice his life for something larger than himself.