The Reign of Terror resulted from several factors. The first was the seizure of the National Convention by the so-called "Mountain," the radicals who were backed by Parisian sans-culottes, and favored a number of radical measures. Most of these measures had to do with conducting the ongoing war with the other powers of Europe, as well as a bloody anti-revolutionary insurrection in the Vendée region of France. The Convention formed the Committee of Public Safety to deal with these challenges. They included crushing the revolt, as well as quelling urban protests against inflation, managing the nation's war resources, including conducting a draft. Under the leadership of Maximilien Robespierre, the Committee moved against the enemies, real and perceived, of the Revolution.
Thousands of Frenchmen, including many who were decidedly not anti-revolutionaries, went to the guillotine during the so-called Reign of Terror which lasted from the summer of 1793 until 1794. The Terror came to an end due to a number of reasons. First, the French became more successful in the war; second, Robespierre made a fool of himself in attempting to establish a "Festival of the Supreme Being" that managed to offend almost everyone in Paris; and most important, he went a step too far in executing fellow radical Georges Danton. Robespierre was himself captured and executed in July of 1794, thus ending the Terror.