I would go with Lavish Spending by the royal court.
Versailles had over 30 water fountains? Each were power by hand.
I know financing the American Revolution was expensive, and took several decades to pay back France. America considers it paid particularly from WWI and WWII.
France's debt grew due mostly to the country´s lengthy engagement in the Seven Years’ War (1756–1763) as well as in the American Revolution (1775–1783). Furthermore, maintaining the army and navy was expensive, but of great importance during that unstable period.
Besides, the cost of maintaining the King Louis XVI’s lavish palace at Versailles and the frivolous spending of his queen, Marie-Antoinette, aggravated an already growing debt.
