The federal arsenal in St. Louis, Missouri, was a prize desired by both sides. Given that Missouri was on the fence—it had neither seceded nor affirmed the Union—the arsenal was in danger of a Confederate takeover. Governor Claiborne Fox Jackson (1806–1862) pretended to be neutral, but those who knew him were quite certain he was angling for the right moment to declare for the Confederacy. During early June 1861, several hundred pro-slavery men gathered in St. Louis, naming their encampment for the governor.

Under ordinary circumstances, St. Louis might well have gone to the Confederacy. There was a very unusual Union commander on the scene, however, Captain Nathaniel Lyon. An eccentric—some even said deluded—person, Lyon was absolutely convinced of the need to act. Working with Frank P. Blair Jr. (1821–1875), brother of the very influential Postmaster General Montgomery Blair (1813–1883), Lyon gathered his men and entered Camp Jackson on June 15, 1861. He was completely successful in disarming the pro-slavery men, but as he marched them to a holding place, his men were attacked by a crowd. Who fired first and who shouted insults the loudest will forever remain mysterious, but Lyon and his men killed several people in the crowd, and a pro-slavery demonstration was made the following day. By then, Lyon had succeeded, however. Camp Jackson was broken up, and the federal arsenal was, for the moment at least, secure.