After the fall of Fort Sumter, Hazen became the captain of the 8th US Infantry during the reorganization of the army. By October, he was promoted to colonel of the 41st Ohio Infantry, and by the following spring, he received the rank of brigadier general. He led a brigade into the Battle of Shiloh. He was involved in the battles of Perryville, Stone's River, Chickamauga, Chattanooga, and Knoxville. He served in the campaign against Atlanta, and with Gen. W.T. Sherman in the "March to the Sea." He was transferred to the Carolinas toward the end of the war as commander of the 2nd Division of the XV Corps.