Pierre Barrois became a French division commander during the Napoleonic Wars. He joined a volunteer battalion in 1793 that later became part of a famous light infantry regiment. He fought at Wattignies, Fleurus, Aldenhoven, Ehrenbreitstein and Neuwied in 1793–1797. He fought at Marengo in 1800. He became colonel of a line infantry regiment in 1803 and led it at Haslach, Dürrenstein, Halle, Lübeck and Mohrungen in 1805–1807. Promoted to general of brigade, he led a brigade at Friedland in 1807.

The Battle of Courtrai saw Johann von Thielmann's Kingdom of Saxony troops and a few Prussians encounter an Imperial French force under Nicolas Joseph Maison near Kortrijk (Courtrai), a city south-west of Ghent in what is now Belgium. Thielmann attacked only to find himself facing the bulk of Maison's I Corps. The action ended in a rout of the Saxons, most of whom were under fire for the first time.