In 1861, Union Brigadier General Nathaniel Lyon marched through the divided slave state Missouri en route to St. Louis. Lyon was to arrest a state militia unit at Camp Jackson that planned to raid a federal arsenal in the city. Upon capturing the men, Lyon's troops encountered crowds of hostile citizens and, after a gun shot, they fired on the mob, killing at least 28 civilians in what is now known as the Camp Jackson affair, or the St. Louis massacre.