Kammala wannan jumla: Wani ɗan asalin New York, Belknap ya kammala karatu daga Jami'ar Princeton a 1848, ya yi karatun doka tare da lauya na Georgetown, kuma ya wuce bar a 1851. Ya koma Iowa, inda ya yi aiki da doka tare da Ralph P. Lowe. Belknap ya shiga siyasa a matsayin dan Democrat kuma ya yi aiki a cikin majalisar wakilan Iowa. Lokacin da yakin basasar Amurka ya barke a 1861, Belknap ya shiga rundunar Union. Wani mutum mai tsayi, Belknap ya kasance jagoran jagorancin Sojojin Union da recruiter. Wani tsohon soja na Iowa Militia wanda ya kai matsayin kyaftin din, an ba shi izini a matsayin babban jami'in soja a cikin 15th Iowa Volunteer Infantry. Ya shiga cikin alƙawura da yawa, ciki har da
Shiloh da Koranti, sun yi aiki a matsayin kwamandan runduna, brigade, division, da kuma kwamandan kwamandan, kuma sun yi aiki a manyan mukamai. A ƙarshen yakin, an ci gaba da Belknap zuwa brigadier janar na masu sa kai kuma ya sami ci gaba ga babban janar.