Kentucky was a southern border state of key importance in the American Civil War and had strong ties to both sides. Governor Beriah Magoffin, favored the right of secession, but sought to find a compromise to avoid it. Most Kentuckians favored John J. Crittenden's position that the Commonwealth should act as a mediator between the two sides. To that end, both houses of the General Assembly passed declarations of neutrality, a position officially declared by Governor Magoffin on May 20, 1861. The August 5, 1861 election for state legislators resulted in veto-proof Unionist majorities of 75–25 in the House and 27–11 in the Senate and the legislature officially voted to declare Kentucky's neutrality in the conflict.
In early September 1861, Confederate General Leonidas Polk invaded Columbus in an attempt to take the state for the south. In response, Union General Ulysses S. Grant entered Paducah a few days later, and Kentucky's neutrality was broken. Governor Magoffin denounced both sides for violating the Commonwealth's neutrality, and called for both sides to withdraw. However, on September 7, 1861, the General Assembly passed a resolution ordering the withdrawal of only Confederate forces. Magoffin vetoed the resolution, but both houses overrode the veto, and Magoffin issued the proclamation.
Notwithstanding the Unionist majority in the legislature, Kentucky's citizens were fairly evenly divided regarding the issues central to the war. But General Polk's invasion created enough hostility that about 75% of the volunteers traveled to the Union camps to enlist. Before the end of September, 2,000 men recruited in McLean and neighboring counties were sent to Calhoun to defend Green River Lock No. 2. They established a camp on a hill overlooking the River. Along with subsequent Union volunteers, they would become the 3rd Kentucky Cavalry, and the 11th, 17th and 25th Kentucky Infantry. Camp Calhoun had suddenly become one of the first military bases in the state and in the following weeks would become arguably the most important.
With the establishment of Camp Calhoun, McLean County men who supported the South needed to go elsewhere in order to enlist with the Confederacy. Many went to the neighboring counties of Webster, Union, Henderson, Daviess, and Hopkins, to join Johnson's 10th Kentucky Partisan Cavalry.