Dickens relied heavily on Thomas Carlyle's The French Revolution when writing A Tale of Two Cities, but he also assumes a certain amount of familiarity with the history of the Revolution. Because the novel is in large measure a historical novel, it is important to be aware of the background against which his characters are acting.
The causes of the French Revolution, which began in 1789, are still debated by historians. Louis XIV had consolidated absolute rule for the French monarchy, and France was governed by the nobility (Monsiegneur's class) and the clergy. The country ran into huge debt due to an archaic system of taxation and the government's financial support of the American Revolution. Because of financial difficulties, the King was forced to call the States-General (the French legislature) in 1789 for the first time since 1614.
The Third Estate (commons) proclaimed themselves the National Assembly and took an oath in a tennis court that they would not disband until they had drawn up a constitution. On July 14, Parisians attacked the Bastille, a symbol of the other two estates (nobility and clergy). The people were mobilized by hunger and fear of retaliation by the nobles and moved to burn down chateaux belonging to noblemen (like Dickens's fictional Monsiegneur's) in what was known as the grande peur ("great fear"). Riots and looting were rampant. A constitution created in 1791 created a limited monarchy with an elected one-body legislature. The king and queen tried to escape but were caught. They returned to Versailles and, humiliated, accepted the constitution. The Jacobin party was on the rise and "Liberty, equality, fraternity" became a catchphrase.
France declared war on Austria, and rumors that the king was guilty of treason turned the people against him. In 1792 a second revolution created the Commune of Paris, which suspended the power of the king and prompted arrests of suspected royalists. The September Massacres occurred thereafter, when mobs murdered 2,000 of these prisoners. The Republic was declared in 1792, and it became increasingly radicalized until Maximilien Robespierre took control and instituted the Reign of Terror, in which many were guillotined including the king and queen. The Revolution drew to a close with the death by guillotine of Robespierre himself in 1794 and the rise to power of Napoleon.