Rubuta ci gaba ga wannan sakin layi - A lokacin da yakin basasa ya barke, Birney ya shiga rundunar soja a ranar 22 ga Mayu, 1861 a matsayin kyaftin din a cikin 1st New Jersey Infantry Regiment. Ya fara ganin yaki a yakin farko na Bull Run. An ci gaba da zama babban jami'in na 4 na New Jersey Volunteer Infantry, ya shiga cikin yakin na biyu na Bull Run, Chantilly
, Fredericksburg, kuma, a matsayin colonel, Chancellorsville. A cikin 1863-64, an nada shi a matsayin ɗaya daga cikin manyan jami'ai uku da ke kula da shiga sojojin launin fata a cikin sojojin Union, kuma a wannan matsayin ya shirya runduna bakwai. An kira shi Colonel na 22 na Amurka Colored Sojoji. A ranar 9 ga Yuni, 1863, an ba shi izini a matsayin brigadier janar na masu sa kai, don darajar daga ranar 22 ga Mayu, 1863, kuma an tura shi zuwa Maryland don karɓar karin sojojin baƙar fata. A shekara ta 1864, ya yi tafiya da rundunarsa don yin yaki a Kudancin Carolina a matsayin wani ɓangare na Ma'aikatar Kudu. Ba su da kyau, amma sun yi aiki mafi kyau a yakin da aka yi a Florida, ciki har da yakin Olustee.