Rubuta ci gaba ga wannan sakin layi - A cikin yakin basasar Amurka, Geddes ya yi aiki a rundunar sojojin Union, ya shiga aikin soja a cikin 8th Iowa Infantry. An ci gaba da shi zuwa kyaftin din kuma daga bisani ya zama mataimakin shugaban kasa, kuma bayan Fabrairu 1862 a matsayin mai kula da masu sa kai. Ya shiga cikin yaƙe-yaƙe a Shiloh
, inda aka raunata shi kuma aka kama shi. An ɗaure shi na ɗan lokaci a Madison, Georgia, da kuma a Kurkuku na Libby a Richmond, Virginia. Bayan an musayar shi, ya yi yaƙi a Vicksburg da Jackson. A watan Oktoba 1863, an ba shi umurnin brigade kuma aka aika zuwa Brownsville, Texas. Daga bisani, ya yi aiki a matsayin babban jami'in tarayya na Memphis, Tennessee, wanda ya kare daga kamawa da dakarun sojan doki na Tarayya a karkashin Nathan Bedford Forrest. Ya umurci brigade a cikin Mobile Campaign kuma ya yi yaƙi da bambanci a yakin Spanish Fort. Ranar 13 ga watan Janairun 1866, Shugaba Andrew Johnson ya zabi Geddes don nada shi a matsayin babban brigadier janar na masu sa kai, don zama daga ranar 5 ga Yuni, 1865, kuma Majalisar Dattijan Amurka ta tabbatar da nadin a ranar 12 ga Maris, 1866.