Confederate General Leonidas Polk commits a major political blunder by marching his troops into Columbus, Kentucky—negating Kentucky’s avowed neutrality and causing the Unionist legislature to invite the U.S. government to drive the invaders away.

Kentucky was heavily divided prior to the war. Although slavery was prevalent in the state, nationalism was strong and Unionists prevented the calling of a convention to consider secession after the firing on Fort Sumter, South Carolina, in April 1861. Governor Beriah Magoffin refused to send troops to either side, and a special session of the legislature in the summer of 1861 issued a warning to both the Confederate and Union armies not to deploy forces in the state. Union and Confederates alike recognized the folly of entering Kentucky into the war, as it would tip the delicate political balance to the other side.

President Abraham Lincoln, a Kentucky native who carefully observed the state’s neutrality, soon realized that the Confederates were acquiring resources and recruiting troops from the state. However, in three special elections held that summer, the Union cause had gained support. Kentucky’s geographic location made permanent neutrality nearly impossible. The major rivers of the upper south drained into the Ohio River through Kentucky, and the state had the country’s ninth largest population.

Troops from both sides began to build fortifications along the border in the opening months of the war, but the Confederates made a critical blunder when General Polk occupied Columbus, Kentucky, on September 4. This preemptive move against the forces of General Ulysses S. Grant, who waited across the Ohio River in Illinois, proved costly for the Confederates. Kentucky’s Unionist legislature invited Federal troops in to drive away the invaders, and on September 6, Grant occupied Paducah and Southland, at the mouths of the Tennessee and Cumberland Rivers, respectively. These were vital positions that allowed the Union a tremendous advantage in the contest for Kentucky and Tennessee.

During the war, some 50,000 white and 24,000 Black Kentuckians fought for the North, while 35,000 joined the South.