How did the french revolution lead to the war with European nations?

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.