As generally defined, the Long 19th Century took place from 1789 to 1914, to include the French Revolution and the end of the Third Republic. France endured many traumatic upheavels during that time.
Literature in 19th Century France directly reflected the slow, painful changeover from monarchy to democracy; the rise of Realism and Naturalism competed with Romanticism, which was the overriding style at the time.
Romanticism attempted to bring moods of nostalgia and reflection to the world as defined by memories of "the old days;" Victor Hugo and Alexandre Dumas were two of the most important Romanticists. These stories were written to show the world "as it once was and should be," rather than "as is."
Realism, which was slower to take hold, rejected the nostalgia of Romanticism to focus on everyday life in situations the common man could identify with. Realism took its cues not from major events, but from simple ones; Honoré de Balzac and Eugéne Sue were important Realists. These stories showed the world "as is," and focused on "realistic" depictions of man's weaknesses.
Naturalism competed directly with Realism in that it depicted the common man in struggles against a harsh world; by using scientific terms and eliminating romance and broad drama, Naturalist writers showed the painful underside of human life. Guy de Maupassant and Emile Zola are often associated with Naturalism.
The many wars -- including several revolutions and the start of World War I -- were a deciding factor in writing, and were often alluded to if not directly involved in the stories. Although some writers took the horrors of war in a vocational, dramatic sense, others denied them altogether, while still others used them to show the innate brutality or impartiality of common life.