Kammala jumla mai zuwa: Dalibin shari'a, ya fara aikin soja a lokacin yana da shekaru goma sha takwas, bayan kafa junta a 1810, wanda ya fara aiwatar da 'yancin kai a New Granada. Santander ya shiga cikin sojojin juyin juya hali a watan Oktoba na 1810, a cikin rundunar soja na National Guard ("Guardias Nacionales"). Da farko ya yi aiki a matsayin soja a rundunar sojojin Tarayyar Tarayyar Tarayyar New Granada, a karkashin jagorancin Janar Antonio Baraya, wanda ya yi yaƙi da Janar Antonio Nariño, na lardin Cundinamarca, wanda ya ƙi amincewa da ikon Tarayyar. A lokacin waɗannan kamfen ɗin Santander ya sami mukamin Kanar a 1812. Bayan da
sojojin sarauta sun sake cinye New Granada, Santander kamar sauran jami'an New Granada da yawa sun koma filayen da ke gabashin Cordillera Oriental, Llanos, kusa da iyakar Venezuela ta zamani. A can, Santander ya haɗu da sojojin Venezuela masu kishin ƙasa kuma ya yi aiki a ƙarƙashin umurnin Simón Bolívar. A lokacin yakin soja a fadin Andes, Bolívar ya inganta Santander zuwa Brigadier Janar a 1817.