Freikorps were irregular German and other European military volunteer units, or paramilitary, that existed from the 18th to the early 20th centuries. They effectively fought as mercenaries or private military companies, regardless of their own nationality. In German-speaking countries, the first so-called Freikorps were formed in the 18th century from native volunteers, enemy renegades, and deserters. These sometimes exotically equipped units served as infantry and cavalry ; sometimes in just company strength and sometimes in formations of up to several thousand strong. There were also various mixed formations or legions. The Prussian von Kleist Freikorps included infantry, jäger, dragoons and hussars. The French Volontaires de Saxe combined uhlans and dragoons.

Ferdinand Baptista von Schill was a Prussian major who revolted unsuccessfully against French domination of Prussia in May 1809.