Rubuta ci gaba ga wannan sakin layi - Lokacin da yakin basasar Amurka ya fara a 1861, Fesler ya kasance tare da Union. A ranar 30 ga Agusta, 1861, ya shiga cikin rundunar sojojin Union a Morgantown, Indiana. Fesler ya zama Kyaftin na Kamfanin G a cikin 27th Indiana Regiment. Fesler ya umurci kamfaninsa a yakin farko na Winchester
da kuma a yakin Cedar Mountain. A shekara ta 1862, an ba da Fesler's Regiment zuwa Rundunar Sojoji ta 12, wanda zai kasance a cikin 1863. Fesler ya umurci kamfaninsa a lokacin yakin Antietam, kusa da Sharpsburg, Maryland, a ranar 17 ga Satumba, 1862. Rundunarsa ta shafe hunturu na 1862 zuwa 1863 a sansanin a Stafford Courthouse, Virginia, kuma a ranar 13 ga Fabrairu, 1863, an ba Fesler mukamin Laftanar Kanar na 27 na Indiana. A watan Maris na 1863, 12th Corps, wanda 27th Indiana ya kasance wani ɓangare, an bayar da tauraron a matsayin alamar ta. A ranar 5 ga Mayu, 1863, Fesler ya ji rauni a lokacin yakin Chancellorsville, a Spotsylvania County, Virginia. Sa'an nan kuma ya umurci dan lokaci na 27th Indiana daga Yuli 2-3, 1863 a lokacin yakin Gettysburg lokacin da aka sanya colonel na rundunar a cikin umurnin brigade. Fesler da rundunarsa sun shiga cikin murkushe Rikicin Draft na Birnin New York a ƙarshen wannan watan.