Kammala jumla mai zuwa: Boniface de Castellane ya shiga rundunar Faransa a ranar da aka naɗa Napoleon I na Faransa (Disamba 2, 1804) a matsayin soja mai ba da izini a cikin Rukunin Sojoji na 5. A watan Fabrairun 1806 an daukaka shi zuwa na biyu a cikin 7th Dragoons. A wannan watan an canza shi zuwa 24th Dragoons wanda ya yi aiki tare da shi a Masarautar Naples. A cikin 1808 de Castellane ya bi janar Georges Mouton zuwa Spain a matsayin "aide-de-camp". Lokacin da Napoléon ya koma Jamus a 1809, de Castellane ya biyo baya kuma ya yi yaƙi a wannan yakin a
Abensberg, Eckmühl, Ratisbon, Aspern-Essling da kuma Wagram.