Rubuta ci gaba don wannan sakin layi - A lokacin da yakin basasa ya barke Osterhaus an nada shi babban jami'in Missouri na 2 na 'yan sa kai na Missouri kuma a farkon shekarar yakin ya yi aiki a Missouri da Arkansas, inda ya dauki wani ɓangare mai ban mamaki a yakin Wilson's Creek da kuma yakin Pea Ridge. A Pea Ridge ya umurci sojojin da suka fara hulɗa da sojojin Confederate da ke ci gaba a hagu na Union. An ci gaba da shi zuwa brigadier janar a ranar 9 ga Yuni, 1862. A shekara ta 1863 ya jagoranci wani rukuni a yakin Port Gibson.
, inda ya nuna kwarewar da ya dace wajen yin amfani da masu kare kariya na Confederate daga matsayi mai kyau.