|Abstract||Historians have long debated the degree to which the Civil War transformed the lives of African-Americans. No African-Americans were more powerfully drawn into the vortex of the Civil War's rapidly changing circumstances that those former Mississippi slaves who became members of the United States Colored Troops (USCT). This dissertation argues that the freedmen who joined USCT in the Middle Mississippi Valley cast aside their plantation masters for a new master in the form of the U.S. Army, but subsequently became masters themselves as they gained experience, only to lose their status after the war ended. When General Ulysses S. Grant began his final campaign for Vicksburg, Mississippi, thousands of slaves fled their masters and attached themselves to his army. President Lincoln saw the Freedmen as a resource that could help turn the tide of the war, and dispatched Adjutant General Lorenzo Thomas to the Mississippi Valley to recruit freedmen into the army. For the former freedmen, military life was not much different from slavery, for they had exchanged one controlling institution for another, spending long hours drilling, foraging, and building fortifications. Furthermore, as on the plantations, they could suffer corporal punishment, or even death, for misdeeds. Yet, as they gained experience and confidence, they emerged from their garrisons to conduct raids and patrols, and most importantly, win victories, thus exercising mastership over the Middle Mississippi Valley. Their supremacy was short lived, for the conclusion of the war brought many former Confederates home. Defeated, but unbroken, ex-Confederates immediately began waging war, political and real, against the USCT. In the end, the men of the USCT mustered out and returned to work in the fields, returning mastery to their former owners.