The French Revolution, which began in 1789, caused many changes throughout Europe. France built huge armies made up of citizens inspired by patriotism. Germany's old-fashioned professional armies were not prepared for the new age.

From 1792 until 1815, France was almost continually at war with other European states. Much of the fighting involved German states and took place on German soil. By the end of 1806, Napoleon—who had seized control of France in 1799—had taken parts of western Germany, set up dependent states, and destroyed the Holy Roman Empire. Some German states became members of the Confederation of the Rhine, which Napoleon had established in 1806 and which was allied with France.

Between 1795 and 1806, Prussia stayed out of the wars. But Napoleon's threats became too great. In 1806, Prussia declared war on France. Napoleon crushed the Prussian army at the battles of Jena and Auerstadt that same year. As a result, Prussia lost its territories west of the Elbe River and had to pay war damages to France. To recover from this defeat, the Prussian government introduced reforms, including laws that freed the serfs and gave some self-government to the cities. In the army, reformers fired incompetent officers and improved military training.

After the failure of Napoleon's Russian campaign in 1812, Austria, the United Kingdom, Prussia, and Russia joined against him. The reformed Prussian army helped defeat Napoleon at Leipzig in 1813 and at Waterloo in 1815.