During the Civil War, the State of Indiana ranked 5th (behind New York, Pennsylvania Ohio, and Illinois) in total soldiers serving in the Union military.
According to the 1860 U.S. census, Indiana, a free state, had a population of 1,350,428.
The Union Army, vol. 3, states, "The contribution of men from the state of Indiana to the military service of the United States from the beginning of the war to Jan. 1, 1865, after which date no further calls were made, was as follows, according to the official report of the adjutant-general: Commissioned officers at original organization, 6,293; non-commissioned officers and musicians at original organization, 1,112; enlisted men, privates, at original organization, 137,401; recruits, privates, 35,836; unassigned recruits, regular army, etc., 16,007; total, 196,649; re-enlisted veterans, 11,718; grand total, 208,367. Of these, 24,418 were killed or died of disease, 10,846 deserted, and 13,779 were unaccounted for. During the war the following numbers of organizations were raised in the state and mustered into the service of the United States for various periods: Cavalry — for three years' service and over, 3 regiments; for three years' service, 10 regiments; for one year's service, 1 company; total, 13 regiments and 1 company. Heavy artillery — for three years' service and over, 1 regiment. Light artillery — for three years' service and over, 11 batteries; for three years' service, 14 batteries; for one year's service, 1 battery; total, 26 batteries. Infantry — for three years' service and over, 40 regiments; for three years' service, 42 regiments; for three years' service, 1 regiment colored troops; for one year's service, 18 regiments and 5 companies; for six months' service, 4 regiments; for one hundred days' service, 8 regiments; for three months' service, 8 regiments; for sixty days' service, 6 companies; for thirty days' service, 2 regiments and 5 companies; total, 123 regiments and 16 companies. Grand total — 137 regiments, 17 companies and 26 batteries. The total number of troops furnished by the state for all terms of service exceeds 200,000 men, much the greater portion of them being for three years; and in addition thereto not less than 50,000 state militia have from time to time been called into active service to repel rebel raids and defend our southern border from invasion." See also The Union Army: Indiana in the American Civil War (1861-1865).
Indiana was the first state in what was then considered the American Northwest to mobilize for the Civil War. News of the attack on Fort Sumter, which began the war, reached Indiana on April 12, 1861. On the next day, two mass meetings were held in the state and the state's position was decided: Indiana would remain in the Union and would immediately contribute men to suppress the rebellion. On April 14, Governor Morton issued a call to arms in order to raise men to meet the quota set by President Abraham Lincoln. Indiana had the fifth-largest population of any state that remained in the Union, and was important for its agricultural yield which became even more valuable to the Union after the loss of the rich farmland of the South. These factors made Indiana critical to the Union's success.