The Camp Jackson affair, also known as the Camp Jackson massacre, occurred during the American Civil War on May 10, 1861, when a volunteer Union Army regiment captured a unit of secessionists at Camp Jackson, outside the city of St. Louis, in the divided slave state of Missouri. The newly appointed Union commander in Missouri, Brigadier General Nathaniel Lyon, had learned that the ostensibly neutral state militia training in Camp Jackson was planning to raid the federal arsenal in St. Louis. After capturing the entire unit, Lyon marched the captives into town to parole them. En route, hostile secessionist crowds gathered, and after an accidental gunshot, Lyon's men fired into the mob, killing at least 28 civilians and injuring dozens of others. Several days of rioting throughout St. Louis followed. The violence ended only after martial law was imposed and Union regulars were dispatched to the city. Lyon's actions ensured Union control of St. Louis and Missouri for the rest of the war but also deepened the ideological divisions within a state that had initially sought to remain neutral in the larger conflict. Background Missouri was a slave state, and many of its leaders were Southern sympathizers who favored secession and joining the Confederacy when the Civil War began. However, there were relatively few enslaved persons and enslavers in Missouri, and only a minority initially favored secession. Nevertheless, by early 1861, both pro- and anti-secession factions in Missouri were organizing military and paramilitary forces. Secessionists organized as "Minutemen", often assisted by state officials. On February 13, Brigadier General Daniel M. Frost enrolled five companies of St. Louis-area Minutemen as the new 2nd Regiment of the Missouri Volunteer Militia. That same month, a new law banned militia activity outside the framework of this militia, forcing pro-Union activists to organize in secret. On February 28, Missouri elected a Constitutional Convention to amend the state constitution and decide the issue of secession. On March 21, the Convention voted 98 to 1 against secession but also voted not to supply weapons or men to either side if war broke out. The Convention then adjourned. On April 20, several days after the Battle of Fort Sumter, a pro-Confederate mob seized the Liberty Arsenal in Liberty, Missouri, and expropriated about a thousand rifles and muskets. This sparked fears that Confederates would also seize the much larger St. Louis Arsenal, which had nearly 40,000 rifles and muskets