The French lose World War I and descend into Civil War, the Bonapartist faction is able to secure an alliance with Britain, America or perhaps even Germany to get badly needed supplies and seizes control of the country. The current Bonapartist pretender is crowned Emperor of the Third Empire.
Alternatively, once defeating France in 1940, Hitler decides that he is the new Napoleon of Europe and to pay homage to Napoleon he has the Bonapartist pretender crowned Emperor. The French however resist war with the allies, and secretly works with the western allies. In 1944 the French Army assists D-day by betraying the Germans, and quickly liberating their nation. {Unlikely I know, the Bonapartists were active in the French Resistance, and are not likely to collaborate, but stranger things have happened then this scenario}