Following the secession of seven southern states, on April 15, 1861 President Lincoln called for 75,000 volunteers to help preserve the Union. Four days later, Bierce enlisted as a private in the Ninth Indiana Infantry, Company C. After three months’ service, and his first battle at Philippi, he re-enlisted as a sergeant and served with his unit in West Virginia.

Bierce fought in some of the most famous and horrific battles of the Civil War, including Shiloh, Corinth, Stones River, Chickamauga, Missionary Ridge, and Franklin. He was commissioned second lieutenant in 1862, and first lieutenant after Stones River in 1863.

 

    

Bierce was in time assigned to the staff of General W. B. Hazen, on which he served as a topographical engineer. In this capacity, he surveyed the landscape and prepared detailed maps of the regions over which the Union army maneuvered and fought.

During the fighting at Kennesaw Mountain on June 23, 1864, Bierce suffered a grievous head wound from a Confederate bullet. Hospitalized for months, he was back in action in September during the Franklin-Nashville campaign. Bouts of dizziness and frequent blackouts -- the aftershocks of his wound -- forced him to resign from the army on January 25, 1865.

Following his departure from the military, Bierce became a Treasury agent in Alabama. In 1866, he joined Hazen on an expedition into Indian Territory, making maps of the regions through which they passed. The party reached San Francisco in 1867, and Bierce remained there as an employee of the U.S. Mint.