IOTL, the Battle of Sedan, during the Franco-Prussian War, was running pretty badly for the french imperial forces, which were being grinded down by the prussians.
Marshall MacMahon advised Napoleon III, who was commanding the army in person, to retreat to Paris and take a defensive stance. Napoleon III, however, rejected this proposal on the basis that it'd be a blow to his prestige, and instead tried striking again at the prussian army. As expected, he failed miserably -- his army, along himself, was encircled, and a surrender was ordered.
News of the emperor's surrender at Sedan reached the rest of France, and led to large unrest. His reputation was blown away. And the rest (Third Republic, Adolphe Thiers, Siege of Paris, Paris Commune) followed.
But what if Napoleon III had listened to MacMahon, and chose to retreat from Sedan into Paris?
With the emperor there to defend and keep Paris in order, what are the ramifications?
Could the Second French Empire survive? With France's defeat probably being less catastrophic, could Alsace and Lorraine remain in french hands?