After Confederate forces occupied Bowling Green on September 18, 1861, young men joined Home Guard units across Kentucky, and before the end of the month 2,000 men recruited in McLean and neighboring counties arrived in Calhoun to defend Green River Lock No. 2. They established a camp on a hill overlooking the Green River about 1/4 mile west of the site. Control of the Green River and its major tributary, the Big Barren, was vital because, navigable from Bowling Green to the Ohio River, they formed an important trade route. When Confederates soon seized Lock No. 3 at Rochester, Union forces rushed to secure the remaining locks, and the Green River became the dividing line between Union and Confederate forces in Kentucky. Near the end of September several Indiana infantry regiments arrived at the camp, bringing the total number of men to more than 5,000. In November, General Thomas L. Crittenden arrived with his command after being appointed commander of Camp Calhoun. In late January more troops arrived in readiness for the Union advance to be spearheaded by General Ulysses S. Grant's assaults on Forts Henry and Donelson, and the camp then housed between 10,000 and 15,000 men, while the civilian population of Calhoun was little more than 500. In early February 1862, most of the troops left to take part in Grant's invasion of Tennessee and the battles at Fort Henry and Fort Donelson, and other regiments soon departed to advance on Mill Springs, Bowling Green, and Nashville. The Confederates then abandoned Kentucky and the focus of the war shifted south, but the Green River region remained dangerous as rebel raiders and guerilla forces, including Sue Mundy, continued to operate, so a small force remained to counter the threat. The last state Guard unit left in late February 1865, which corresponds to the last known burial at Camp Calhoun Cemetery. Thomas Crittenden was the son of Kentucky statesman John Crittenden, author of the Crittenden Compromise, an attempt to find a peaceful solution to differences that seemed destined to end in civil war, and the family's divided loyalties were reflected in the war when Thomas's brother George joined the Confederate army and also became a general.