The Napoleonic Wars
After its first, surprise victory over the Prussians at Valmy in 1792, the French army announced it would come to the defense of "all peoples who wish to recover their liberty". This meant that it would not only defend France from invasion but help overthrow kings in other countries.
This policy was most successful in Belgium (to the north of France), and in northern Italy (to the south of France), which had been controlled by the King of Austria. Between 1793 and 1795, the French army invaded these areas and helped them establish "sister republics" that were allied with France. The French believed that free peoples would not make war on each other.
These successes led the other European countries to fear that France would try to "export" its Revolution, and in 1797, Britain, Prussia, and Austria formed a Coalition against it. (Later, Russia would also join this Coalition.) When Napoleon became Emperor in 1799, he hoped to defeat the Coalition as a way to establish stability in France and end the Revolution.
Between 1801 and 1810, he won a series of battles against Austria, Prussia, and Britain, after which most of the European continent was either annexed into or allied with France. The most significant result of these victories was in 1806, when after having defeated Prussia at Jena, Napoleon dissolved the Holy Roman Empire and combined much of German-speaking Europe into a new country, known as the Confederation of the Rhine. Napoleon introduced the Napoleonic Code, the Prefectural police system and the draft; in response, most of the people began to view Napoleon and the French not as "liberators" but as occupiers. (Some German-speakers began an effort to force the French out by suggesting that all German-speaking people unite into a single German country, led by the King of Prussia.)