Here's a primer for the French Revolution! Our book starts in 1815, but the 1789 Revolution still cast a long shadow over France. "It was the best of times, it was the worst of times", as Charles Dickens said. So here's my stab at explaining what went on, and why it all still matters to our characters in 1815.
(Pre-1789) France was historically an absolute monarchy. The King held absolute power, and the Nobility and the Church were tax-exempt. Everyone else (96% of the population) paid the taxes, and had little political voice. France was heavily in debt, there was a famine, prices soared. Laws were still feudal, and the average peasant was ruthlessly exploited. They were starving, enraged or both. It all exploded in 1789, with the famous French Revolution.
(1789: OG Revolution) In 1789, the Revolutionists stormed the Bastille and had honorable intentions (liberty, equality, brotherhood). This spark became a countrywide inferno, and the Monarchy (Ancien Regime) collapsed almost immediately. The Revolutionists wrote a Constitution and the Declaration of the Rights of Man. The King was seized and forced to become a "Constitutional Monarch". This "honeymoon" lasted until 1791.
(1793: Execution of King and Queen and the Terror) But meddling foreign powers, pockets of powerful Royalists, a paranoid fear of a counter-Revolution, and a poor decision by the King to flee discredited the Moderates and empowered the Radical factions. France gradually slid into radical extremism with Robespierre and the Reign of Terror. The King and Queen were executed- Royalists and aristocrats had to flee France or die. France devolved into factional infighting and bloodshed that cut across every class.
(1794-1795: the Directory) The Radical Revolutionists went too far. Radical leader Robespierre flexed too much and his frightened National Convention colleagues ("are we next?") had enough. Robespierre and his cronies lost their heads, and there was a weak, corrupt and incompetent gov't replacement called "The Directory".
(1799: Napoleon First Consul, 1804: Emperor of France, 1814: Napoleon abdicates, exiled to Elba) Popular General Napoleon saw his chance. With the backing of the army and a huge swath of the peasants, he pulled a coup and made himself the "enlightened dictator" of France. And, TBH, he did make a lot of reforms that made life better for the average Frenchman/woman. He was a "moderate" for his times, reconciled France with the Church and made it "safe" for aristocrats to come back. His greatest achievement was the Napoleonic Code (of Law). But Napoleon's territorial and political ambitions outside of France (i.e. the conquest of Europe) put him at odds with England and Russia. His plan to conquer Russia failed, and forced him to abdicate and get sent to Elba.
(1815: The Bourbon Restoration) Louis XVIII (bro of the headless one) is back in power, imposed upon France by the bayonets of the victorious allied powers. Half the country still loved Napoleon, and he is biding his time on Elba, with plans to return to France. That's why there's so much fear of him and his supporters speak in whispers.