The "St. Louis Massacre" was an incident in the Civil War that began on May 10, when Union military forces clashed with civilians on the streets of St. Louis, Missouri, resulting in the deaths of at least 28 and injuries of roughly 100.
The events began when Union Capt. Nathaniel Lyon, a Radical Republican known for his brazenness, used a newly mustered force of roughly 3,000 men to arrest a Missouri State Militia encampment located outside of the city. It was widely rumored that the militia intended to take possession of the hotly contested St. Louis Arsenal, which both union and Confederate forces desired.
After surrounding the militia encampment, Lyon decided to march his prisoners through downtown St. Louis before providing them with a parole and ordering them to disperse. This march was widely viewed as humiliation for the state forces and immediately angered citizens who had gathered to watch the commotion. Tensions mounted quickly on the streets as civilians hurled fruit, rocks, and insults at Lyon's troops and some of the soldiers returned the favor.
Nobody knows exactly what happened to provoke the massacre, but the standard report says that a drunkard stumbled into the path of the marching soldiers and got into an altercation with some of them. Weapons were drawn by both the soldiers and civilians and shots rang out. Some of the soldiers formed a line and fired into the nearby crowd. Violence continued for the next two days resulting in the death of at least seven more civilians, who were shot by Union troops patrolling the streets.
The St. Louis Massacre, as it came to be called, quickly sparked an outcry across the state of Missouri. Prior to that point, most Missourians had been moderate unionists who were opposed to secession and war. Popular opinion transformed overnight, causing many former unionists including former Governor Sterling Price to advocate secession and producing a state that was bitterly divided between Union and Confederate sympathizers.