I would guess that during the French revolution France was so poor and miserable that the people revolted and started the french revolution.
WHat I don't understand is how were they able to afford to have and supply the Grand Army commanded by Napoleon? A poor empire I would assume wouldn't be able to afford the most powerful Army in the world.
Several reasons:
France wasn't necessarily poor: the French government was because it had problems generating revenue since that required them to increase taxation on the nobility, who had a large degree of immunity from taxes as part of their privileges. The Revolution did away with those privileges, and in the more radical parts simply confiscated their asset wholesale.
Occupied territories: one of the reasons why French revolutionary armies were so mobile is that they didn't depend as much on supply trains, which mean that they "lived off the country". This translated to looting and stealing from the locals, which meant that France did not have to pay as much for their upkeep. There was also more organized system of forcing "liberated people" to pay for French armies: in Belgium for instance this was done through taxation, in Holland through indemnities. In Italy Napoleon managed to extract a "tribute" of 60 million Francs during the first 3 month of his 1797 campaign.
Rationalization of the internal economy: the revolution did do away with systems of internal tariffs which expanded markets. Traditional guild privileges were abolished, and the new government were somewhat more efficient administrators than the old one.
Source: The French Revolution and the Birth of Modernity by Ferenc Fehér
The French Revolution By George Rude
I'd raise attention to a splendid post by /u/_elos on this subject.