Indiana, a state in the Midwest, played an important role in supporting the Union during the American Civil War. Despite anti-war activity within the state, and southern Indiana's ancestral ties to the South, Indiana was a strong supporter of the Union. Indiana contributed approximately 210,000 Union soldiers, sailors, and marines. Indiana's soldiers served in 308 military engagements during the war; the majority of them in the western theater, between the Mississippi River and the Appalachian Mountains. Indiana's war-related deaths reached 25,028 (7,243 from battle and 17,785 from disease). Its state government provided funds to purchase equipment, food, and supplies for troops in the field. Indiana, an agriculturally rich state containing the fifth-highest population in the Union, was critical to the North's success due to its geographical location, large population, and agricultural production. Indiana residents, also known as Hoosiers, supplied the Union with manpower for the war effort, a railroad network and access to the Ohio River and the Great Lakes, and agricultural products such as grain and livestock. The state experienced two minor raids by Confederate forces, and one major raid in 1863, which caused a brief panic in southern portions of the state and its capital city, Indianapolis.

Indiana experienced significant political strife during the war, especially after Governor Oliver P. Morton suppressed the Democratic-controlled state legislature, which had an anti-war (Copperhead) element. Major debates related to the issues of slavery and emancipation, military service for African Americans, and the draft, ensued. These led to violence. In 1863, after the state legislature failed to pass a budget and left the state without the authority to collect taxes, Governor Morton acted outside his state's constitutional authority to secure funding through federal and private loans to operate the state government and avert a financial crisis.

The American Civil War altered Indiana's society, politics, and economy, beginning a population shift to central and northern Indiana, and contributed to a relative decline in the southern part of the state. Increased wartime manufacturing and industrial growth in Hoosier cities and towns ushered in a new era of economic prosperity. By the end of the war, Indiana had become a less rural state than it previously had been. Indiana's votes were closely split between the parties for several decades after the war, making it one of a few key swing states that often decided national elections. Between 1868 and 1916, five Indiana politicians were vice-presidential nominees on the major party tickets. In 1888 Benjamin Harrison, one of the state's former Civil War generals, was elected president of the United States.

Indiana was the first of the country's western states to mobilize for the Civil War. When news reached Indiana of the attack on Fort Sumter, South Carolina, on April 12, 1861, many Indiana residents were surprised, but their response was immediate. On the following day, two mass meetings were held in Indianapolis, the state capital of Indiana, and the state's position was decided: Indiana would remain in the Union and would immediately contribute men to suppress the rebellion. On April 15, Indiana's governor, Oliver P. Morton, issued a call for volunteer soldiers to meet the state's quota set by President Abraham Lincoln.

Indiana's geographical location in the Midwest, its large population, and its agricultural production made the state's wartime support critical to the Union's success. Indiana, with the fifth-largest population of the states that remained in the Union, could supply much-needed manpower for the war effort, its railroad network and access to the Ohio River and the Great Lakes could transport troops and supplies, and its agricultural yield, which became even more valuable to the Union after the loss of the rich farmland of the South, could provide grain and livestock.

On April 15, 1861, President Lincoln called for a total of 75,000 volunteers to join the Union army. On the same day, Governor Morton telegraphed the president offering 10,000 Indiana volunteers. The state's initial quota was set at six regiments (a total of 4,683 men) for three months of service. Orders were issued on April 16 to form the state's first regiments and to gather at Indianapolis. On the first day, five hundred men were encamped in the city; within a week more than 12,000 Hoosier volunteers had signed up to fight for the Union, nearly three times as many needed to meet the state's initial quota.

Governor Morton and Lew Wallace, Indiana's adjutant general, established Camp Morton at the state fairgrounds in Indianapolis as the initial gathering place and training camp for the state's Union volunteers. (Camp Morton was converted to a prisoner-of-war camp in 1862.) By April 27, Indiana's first six regiments were fully organized as the First Brigade, Indiana Volunteers, under the command of Brigadier General Thomas A. Morris. Members of companies not selected for these first regiments were given the option of volunteering for three years of service or returning home until they were needed; some companies formed into regiments in the state militia and were called into federal service within a few weeks.

Indiana ranked second among the states in terms of the percentage of its men of military age who served in the Union army. Indiana contributed 208,367 men, roughly 15 percent of the state's total population to serve in the Union army, and 2,130 to serve in the navy. Most of Indiana's soldiers were volunteers; 11,718 were re-enlistments. Deserters numbered 10,846.