On July 1, 1862, a full year after the bombardment of Fort Sumter, President Abraham Lincoln appealed for an additional 300,000 men in the Union Army. Along with 17 other governors, Governor Oliver P. Morton of Indiana pledged to aid in the call for reinforcements. In response to this pledge, Benjamin Harrison volunteered to raise a regiment. He was immediately commissioned a Lieutenant and two weeks later was promoted to captain. By August 8, 1862 the newly appointed Colonel Harrison had raised 1000 men known as the 70th Indiana Regiment.\
From November 1862, to March 1864, the 70th Indiana was stationed at various posts in western Kentucky and Tennessee. Beginning in May 1864, Colonel Harrison and the regiment joined General Sherman’s Atlanta campaign in the Army of the Cumberland. For Harrison’s achievements at the battles of Resaca and Peachtree Creek, he was promoted to Brigadier General.
After the fall of Atlanta to the Union forces, Harrison received orders to report to Governor Morton for special duty. The next several weeks in Indianapolis were spent campaigning both for himself as Indiana’s Supreme Court Reporter and President Abraham Lincoln.
After the November election, he left for Georgia to rejoin his old regiment for Sherman’s "March to the Sea." Instead he was given command of the 1st Brigade at Nashville and led them in a decisive battle against Confederate General Hood.
On April 29, 1865, the regiment was ordered to march to Washington, D.C. It participated in the Grand-Review of Western Armies held on May 24, 1865. General Harrison and the 70th Indiana were mustered out of Federal service on June 8, 1865.