The word that best completes the sentence is "Enlightenment".
The Enlightenment was a philosophical movement in the 18th century that emphasized reason, individualism, and rationality. It had a profound impact on European intellectual and cultural life, and its ideals spread throughout Europe and the Americas. The French Revolution was a major event that was influenced by the Enlightenment, and it led to the overthrow of the French monarchy and the establishment of a republic based on the principles of liberty, equality, and fraternity. The sentence suggests that the spread of Enlightenment ideals was one of the factors that contributed to the widespread unrest and revolt against conservative imperial powers.