Volunteer ranks complicated command hierarchy

The Union order of battle appears crowded with major generals and brigadier generals serving in uneven roles. Congress was reluctant to create a large permanent senior-officer establishment, partly from fear of military aristocracy and partly from cost. Officers could therefore hold one rank in the regular army and another in the volunteers, with substantive and acting status adding further complexity. The arrangement complicated seniority, promotion, and the reading of command relationships. Similar ambiguity persisted into the First World War before the United States developed a more regular senior-career structure.