On September 13, 1861, Union and Confederate forces fought the Second Battle of Boonville, roughly halfway in Missouri between St. Louis and Kansas City. It was an otherwise forgettable battle in which 800 Confederate militia under Col. William Brown unsuccessfully tried to surprise a local Unionist militia of only 140 men. The Confederates, protecting their flags against the rain, covered them in black erroneously convincing the Unionists that they intended to give no quarter. Hence, the entrenched Unionists put up a determined resistance despite being badly outnumbered, convinced they were literally fighting for their lives and actually ended up winning the battle, repulsing the Confederate assault.
One of the reasons they won was because they had help from the rebels’ slaves. Buried in the endnotes of Michael Fellman’s Inside War: The Guerrilla Conflict in Missouri During the American Civil War is an interesting excerpt of a letter from a Unionist soldier, Daniel R. Smith. Writing his parents on September 17, 1861, he related:
Five negroes brought word of the intended attack and were inside the entrenchment during the engagement. Among them was the slave of Col. Brown’s [the Confederate commander]. Whilst fighting he took hold of a gun and shot his master who fell and soon after expired. The darky is tickled almost to death.
So were these slaves soldiers? Strictly speaking, no, as they were not enlisted members of formal military unit. They appear to have escaped their owners in the rebel force and sought refuge with the Union militia, bringing them a warning of the impending attack. During the battle, like the Unionists, with nowhere to escape and convinced they would be killed if captured, they fiercely defended themselves. Col. Brown’s slave was lucky to arm himself as his owner approached and exact a personal vengeance. So the slaves’ participation in the Second Battle of Boonville, although informal, for them was a matter of life and death. So they were soldiers there in a viscerally real way, fighting an impromptu battle for their lives. Their service, such as it was, is reminiscent of other incidents during Summer 1861 documented in earlier editions of Civil War Emancipation. See: 1) July 27; 2) July 28; 3) August 1; 4) August 17.
Source: Michael Fellman, Inside War: The Guerrilla Conflict in Missouri During the American Civil War (New York: Oxford University Press, 1989), 280-81.