Several historians consider the "reign of terror" to have begun in 1793, placing the starting date at either 5 September, June  or March (birth of the Revolutionary Tribunal), while some consider it to have begun in September 1792 (September Massacres), or even July 1789 (when the first beheadings by guillotine took place), but there is a consensus that it ended with the fall of Maximilien Robespierre in July 1794.
There was a sense of emergency among leading politicians in France in the summer of 1793 between the widespread civil war and counter-revolution. Bertrand Barère exclaimed on 5 September 1793 in the Convention: "Let's make terror the order of the day!" They were determined to avoid street violence such as the September Massacres of 1792 by taking violence into their own hands as an instrument of government.
Robespierre in February 1794 in a speech explained the necessity of terror:
If the basis of popular government in peacetime is virtue, the basis of popular government during a revolution is both virtue and terror; virtue, without which terror is baneful; terror, without which virtue is powerless. Terror is nothing more than speedy, severe and inflexible justice; it is thus an emanation of virtue; it is less a principle in itself, than a consequence of the general principle of democracy, applied to the most pressing needs of the patrie [homeland, fatherland].
Enlightenment thought emphasized the importance of rational thinking and began challenging legal and moral foundations of society, providing the leaders of the Terror with new ideas about the role and structure of government.Rousseau's Social Contract argued that each person was born with rights, and they would come together to form a government that would then protect those rights. Under the social contract, the government was required to act for the general will, which represented the interests of everyone rather than a few factions. Drawing from the idea of a general will, Robespierre felt that the French Revolution could result in a Republic built for the general will but only once those who fought this ideal were expelled. Those that resisted the government were deemed "tyrants" fighting against the virtue and honor of the general will. The leaders felt their ideal version of government was threatened from the inside and outside of France, and terror was the only way to preserve the dignity of the Republic created from French Revolution.
During the Reign of Terror, the sans-culottes and the Hébertists put pressure on the National Convention delegates and contributed to the overall instability of France. The National Convention was bitterly split between the Montagnards and the Girondins. The Girondins were more conservative leaders of the National Convention, while the Montagnards supported radical violence and pressures of the lower classes. Once the Montagnards gained control of the National Convention, they began demanding radical measures. Moreover, the sans-culottes, the scrappy, urban workers of France, agitated leaders to inflict punishments on those who opposed the interests of the poor. The sans-culottes' violent demonstrations pushing their demands, created constant pressure for the Montagnards to enact reform. The sans-culottes fed the frenzy of instability and chaos by utilizing popular pressure during the Revolution. For example, the sans-culottes sent letters and petitions to the Committee of Public Safety urging them to protect their interests and rights with measures such as taxation of foodstuffs that favored workers over the rich. They advocated for arrests of those deemed to oppose reforms against those with privilege, and the more militant members would advocate pillage in order to achieve the desired equality. The resulting instability caused problems that made forming the new Republic and achieving full political support even more critical.
The Reign of Terror was characterized by a dramatic rejection of long-held religious authority, its hierarchical structure, and the corrupt and intolerant influence of the aristocracy and clergy. Religious elements that long stood as symbols of stability for the French people, were replaced by reason and scientific thought. The radical revolutionaries and their supporters desired a cultural revolution that would rid the French state of all Christian influence. This process began with the fall of the monarchy, an event that effectively defrocked the State of its sanctification by the clergy via the doctrine of Divine Right and ushered in an era of reason.
Many long-held rights and powers were stripped from the Church and given to the State. In 1789, church lands were expropriated and priests killed and forced to leave France. A Festival of Reason was held in the Notre Dame Cathedral, which was renamed "The Temple of Reason," and the old traditional calendar was replaced with a new revolutionary one. The leaders of the Terror tried to address the call for these radical, revolutionary aspirations, while at the same time trying to maintain tight control on the de-Christianization movement that was threatening to the clear majority of the still devoted Catholic population of France. The tension sparked by these conflicting objectives laid a foundation for the "justified" use of terror to achieve revolutionary ideals and rid France of the religiosity that revolutionaries believed was standing in the way.
On 10 March 1793 the National Convention created the Revolutionary Tribunal. Among those charged by the tribunal, about a half were acquitted (though the number dropped to about a quarter after the enactment of the Law of 22 Prairial). In March rebellion broke out in the Vendée in response to mass conscription, which developed into a civil war that lasted until after the Terror.
On 6 April the Committee of Public Safety was created, which gradually became the de facto war-time government.
On 2 June, the Parisian sans-culottes surrounded the National Convention, calling for administrative and political purges, a low fixed price for bread, and a limitation of the electoral franchise to sans-culottes alone. With the backing of the national guard, they persuaded the convention to arrest 29 Girondist leaders. In reaction to the imprisonment of the Girondin deputies, some thirteen departments started the Federalist revolts against the National Convention in Paris, which was ultimately crushed. On 24 June, the convention adopted the first republican constitution of France, the French Constitution of 1793. It was ratified by public referendum, but never put into force.
On 13 July the assassination of Jean-Paul Marat - a Jacobin leader and journalist - resulted in a further increase in Jacobin political influence. Georges Danton, the leader of the August 1792 uprising against the king, was removed from the committee. On July 27, 1793, Robespierre became part of the Committee of Public Safety.
On 23 August, the National Convention decreed the Levée en masse, "The young men shall fight; the married man shall forge arms and transport provisions; the women shall make tents and clothes and shall serve in the hospitals; the children shall turn all lint into linen; the old men shall betake themselves to the public square in order to arouse the courage of the warriors and preach hatred of kings and the unity of the Republic."
On 9 September, the convention established paramilitary forces, the "revolutionary armies", to force farmers to surrender grain demanded by the government. On 17 September, the Law of Suspects was passed, which authorized the imprisonment of vaguely defined "suspects". This created a mass overflow in the prison systems. On 29 September, the convention extended price fixing from grain and bread to other essential goods, and also fixed wages.
On 10 October, the Convention decreed that "the provisional government shall be revolutionary until peace." On 24 October, the French Republican Calendar was enacted. The trial of the Girondins started on the same day and they were executed on 31 October.
Anti-clerical sentiments increased during 1793 and a campaign of dechristianization occurred. On 10 November (20 Brumaire Year II of the French Republican Calendar), the Hébertists organized a Festival of Reason.
On 14 Frimaire (5 December 1793) was passed the Law of Frimaire, which gave the central government more control over the actions of the representatives on mission.
On 16 Pluviôse (4 February 1794), the National Convention decreed that slavery be abolished in all of France and French colonies.
By the end of 1793, two major factions had emerged, both threatening the Revolutionary Government: the Hébertists, who called for an intensification of the Terror and threatened insurrection, and the Dantonists, led by Georges Danton, who demanded moderation and clemency. The Committee of Public Safety took actions against both. The major Hébertists were tried before the Revolutionary Tribunal and executed on 24 March. The Dantonists were arrested on 30 March, tried on 3 to 5 April and executed on 5 April.
On 20 Prairial (8 June) was celebrated across the country the Festival of the Supreme Being, which was part of the Cult of the Supreme Being, a deist national religion. On 22 Prairial (10 June), the National Convention passed a law proposed by Georges Couthon, known as the Law of 22 Prairial, which simplified the judicial process and greatly accelerated the work of the Revolutionary Tribunal. With the enactment of the law, the number of executions greatly increased, and the period from this time to the Thermidorian Reaction became known as "The Grand Terror".
On 8 Messidor (26 June), the French army won the Battle of Fleurus, which marked a turning point in France's military campaign and undermined the necessity of wartime measures and the legitimacy of the Revolutionary Government.
The fall of Robespierre was brought about by a combination of those who wanted more power for the Committee of Public Safety (and a more radical policy than he was willing to allow) and the moderates who completely opposed the revolutionary government. They had, between them, made the Law of 22 Prairial one of the charges against him, so that, after his fall, to advocate terror would be seen as adopting the policy of a convicted enemy of the republic, putting the advocate's own head at risk. Between his arrest and his execution, Robespierre may have tried to commit suicide by shooting himself, although the bullet wound he sustained, whatever its origin, only shattered his jaw. Alternatively, he may have been shot by the gendarme Merda. The great confusion that arose during the storming of the municipal Hall of Paris, where Robespierre and his friends had found refuge, makes it impossible to be sure of the wound's origin. In any case, Robespierre was guillotined the next day.
The reign of the standing Committee of Public Safety was ended. New members were appointed the day after Robespierre's execution, and limits on terms of office were fixed (a quarter of the committee retired every three months). The Committee's powers were gradually eroded.