De Borre began his military career in the French royal army as a volunteer. He served as a cavalry officer in several campaigns during the War of the Austrian Succession. In 1745 he was seriously wounded. In 1757 he raised a unit from his home town of Liège in what is now Belgium but evidently saw no action in the Seven Years' War. When he arrived in America in 1776, the Second Continental Congress commissioned him a brigadier general. George Washington assigned de Borre to command the 2nd Maryland Brigade in John Sullivan's division. He led his troops at Staten Island and Brandywine. At the latter battle he proved incapable of handling his troops in action. Threatened with a court martial, he resigned and left America in 1779. He was promoted to brigadier general in the French army in 1780 but he was no longer physically capable of active service.