Mgbe Agha Obodo ahụ malitere na 1861, Smith jere ozi dị ka onye inyeaka de camp nye Gọvanọ Illinois Richard Yates. Na Disemba 26, 1861, a họpụtara ya ka ọ bụrụ onye isi nke 45th Illinois Volunteer Infantry Regiment, na ọkwa azụ na July 23, 1861 iji dochie Colonel gara aga nke gbara arụkwaghịm na Kọmitii steeti ya mgbe ndị agha ahụ banyere ọrụ Federal. Ọ duru ndị agha ya n'agha Fort Henry, Fort Donelson na Shiloh. Na November 29, 1862 e buliri ya n'ọkwa brigadier general nke U.S. Volunteers. O duziri otu brigade nwa oge tupu o buru onye isi nke 8th Division, XVI Corps. Mgbe Ulysses S. Grant malitere njem ikpeazụ ya megide Vicksburg, e nyere Smith iwu nke 1st Brigade, 3rd Division, XVII Corps. Ọ lụrụ agha na Port Gibson
, Raymond, na Champion Hill nakwa na mwakpo ndị ahụ a wakporo Vicksburg. Na June, 1863 n'etiti nnọchibido nke Vicksburg, a họpụtara Smith iji dochie Gen. Isaac F. Quinby n'ịchị nke 7th Division, XVII Corps mgbe Quinby dara ọrịa ma were ezumike. N'ọnwa Septemba, 1863, a kpọfere Smith ka ọ chịwa ngalaba nke abụọ, XVII Corps ma zigara ngalaba ya na William T. Sherman iji nyere aka na enyemaka nke Chattanooga. N'oge agha nke Missionary Ridge, Smith sonyere na mwakpo megide ndị agha Confederate n'akụkụ aka nri na Tunnel Hill.