Spears was promoted to brigadier general on March 5, 1862. He was placed in command of the 25th Brigade of the Army of the Ohio. In June 1862, the 25th participated in General George W. Morgan's capture of the Cumberland Gap, a key mountain pass located at the junction of Tennessee, Kentucky and Virginia. In August, Spears led a sortie into Tennessee, and scattered a small Confederate contingent at Wallace's Crossroads (near modern LaFollette). Senator Andrew Johnson had suggested Spears be appointed commander of forces around Cumberland Gap, but Union generals were concerned that the hot-tempered Spears would seek violent retribution against East Tennessee Confederates. General Morgan wrote that Spears had great energy and courage, but has an idea that war means extermination.