France (still a constitutional monarchy) declared war on Prussia and Austria in 1792 for various reasons, among them French concerns about counter-revolutionary emigres in German-speaking countries and in the Austrian Netherlands, and fear that Austria and Prussia might intervene in French affairs (either to support Louis XVI or to take advantage of the internal dissent).
After the execution of Louis XVI, several more states, Great Britain and Spain among them, severed ties. The Republic then declared war on those countries, and a general European conflict ensued in one form or another for the next 22 years.
The Enlightenment or Age of Reason was the predominant philosophical movement in the 18th century. Enlightenment philosophers based their ideas in reason as the main source of power. They were very skeptical of the power of the Catholic Church and monarchies as they believed that science and reason were essential to empower people. The most famous Enlightenment philosophers were Kant, Adam Smith, and Voltaire. The main ideas spread during this period were freedom, tolerance and learning.
The Enlightenment Era started after the Dark Middle Ages and preceded the Romantic Period.