What was the cause of the Reign of Terror during the French Revolution?

The
proximate (most direct) cause of the Terror was the rise to power of the radical Jacobin faction
within the National Convention in 1793. These politicians had the support of the
sans-culottes, the Parisian working class, and they were able to use this
broad base of political support to prevail over moderate factions, sometimes known as the
Girondins. The Jacobins instituted radical reforms that included price supports and instituted
the Committee of Public Safety to run the country. The Committee, as well as local revolutionary
councils, famously sent tens of thousands of supposed enemies of the Revolution to their deaths,
most memorably using the guillotine. The Terror did not end until it consumed its own leaders,
as Maximilian Robespierre and others were themselves guillotined in 1794. To understand the
deeper origins of the Terror, however, we must remember that it took place in a time of extreme
crisis for the Revolution. In the words of one historian, the Committee was essentially formed
to serve as a "war cabinet." France was at war with almost all the monarchies of
Europe, and confronted a massive revolt in the Vendee region. Faced with spiraling inflation and
food shortages, setbacks on the battlefield, and mob violence that was completely out of
control, the Terror was in no small part an attempt to establish order (albeit a radical order)
to a Revolution and a nation beset with chaos.

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