Answering the question, "When did the French Revolution start?" is a lot more difficult than simply naming a date or one specific event. It was a string of events and circumstances that led to the French Revolution rather than simply one event or happening.
When Did the French Revolution Start?
Most history textbooks will say that the French Revolution officially began on July 14, 1789 as Parisians stormed the Bastille in search of ammunition and in the hopes of freeing those who had been wrongfully imprisoned. Bastille Day is celebrated on France commemorating the storming of the Bastille and so in many ways, if there is an official answer for 'when did the French Revolution start?', July 14, 1789 could be it. However, there were many factors brewing for several years that led to the storming of the Bastille and true historians would argue that the French Revolution indeed started before that night on July 14.
Enlightenment
The idea that all men should be free and that government should serve the people was truly a revolutionary idea in feudal France. Writers and philosophers that had been writing throughout the 18th century were highly influential on both the Americans and the French. People were just beginning to get a sense of what a fair and just government could truly mean for them.
Bankruptcy
While Louis XVI was known to spend lavishly on parties and other expenses, he truly inherited France's financial problems rather than causing them. France was already on the verge of bankruptcy due to its involvement in the Seven Years War and the American Revolution. Had the financial crisis not been so severe, perhaps there would have been no cause for revolution. However, the nobility, clergy, and courtiers lived well while the rest of France suffered. To deal with the impending financial doom, Louis XVI called together the Estates General on May 5th of 1789. It was here that the largest estate of the three estates in the Estates General, the peasants, recognized their strength in numbers and decided to break off into a sovereign National Assembly and effectively remove Louis XVI from power.
Beginning of the French Revolution
While the official date of the beginning of the French revolution is July 14, it could be argued that the tide truly started changing on May 5th of that same year.
