The purpose of this thesis is to illustrate the importance of the military principle of unity of command by examining the military history of a Union army unit during the Civil War. The Mississippi Marine Brigade and its predecessor, the Ellet Ram Fleet, being a creation of the War Department, and yet conducting tactical operations within the scope of the Navy Department, vividly illustrates the problems inherent in joint army-navy operations. The brigade's primary mission was to counter guerrilla warfare in the Mississippi River valley. The text describes the organization, administration, and major operations of the brigade as a mobile, independent, private military force.