Born in the French colony of Vermont in 1770, Gaspard Lefebvre grew up as an outsider defined by physical ailments and intellectual isolation. Although his family was wealthy, a difficult birth left him with a permanent limp, and he suffered from a severe stutter and frequent illnesses that kept him confined to his home. To cope, the young Lefebvre immersed himself in books, mastering multiple languages by the age of five and developing obsessive interests in astronomy and military history, particularly the exploits of Hannibal. At thirteen, he was sent to France under a fabricated noble pedigree to attend a military academy, where he was relentlessly bullied for his colonial background and speech impediments. Despite these challenges, he managed his emotions through reading and speech therapy, eventually graduating in 1788 to join a light cavalry regiment, where he formed a close bond with his mule, Chloe, and remained detached from the growing political unrest.

As the Revolution dismantled the French state, Lefebvre remained ideologically indifferent, caring only for the cohesion of his military unit. Following the desertion of his royalist commanders in 1792, he wandered until joining a militia division in Picardy, which saw little action due to logistical chaos. However, during the violent anti-Vanguard uprisings in July 1793, Lefebvre spontaneously took command of a disorganized cavalry regiment to crush a hostile force on the outskirts of Paris. Unaware that he had been following orders from counter-revolutionaries, he was subsequently arrested by the victorious Vanguard regime. He spent three months in prison but was released after officials concluded his actions were the result of political naivety rather than treason.

Lefebvre spent the following year aimless and unemployed until the summer of 1794, when the invasion of France by the Habsburgs and Italian states created a desperate need for capable officers. Remembering his tactical efficiency during the Paris revolt, the high command of the Army of the Interior recalled him to service on September 2nd. At just twenty-three years old, the "colonial upstart" was given command of the VI Division near Reims, entrusted with 2,500 cavalrymen and tasked with defending a republic he barely understood against the encroaching armies of Europe.