The Camp Jackson Affair was an incident of civil unrest in the American Civil War on May 10, 1861, when Union military forces clashed with civilians on the streets of St. Louis, Missouri, resulting in the deaths of at least 28 people and injuries to another 100. The highly publicized affair polarized the border state of Missouri, leading some citizens to advocate secession and others to support the Union, thus setting the stage for sustained violence between the opposing factions.