Military units of the Civil War varied greatly in size and composition. Army formations were often fighting under strength due to casualties, sickness, or desertions. In general, however, the following list explains the basic terms and size of Civil War army organization.
Battery: 4–6 cannons commanded by a lieutenant
Troop: 80–100 cavalry commanded by a captain
Company: 100 men commanded by a captain
Regiment: 800–1,000+ men, 10 companies commanded by a colonel
Brigade: 3,000–4,000+ men, 4–6 regiments commanded by a brigadier general
Division: 10,000–12,000 men, 3+ brigades commanded by a brigadier or major general
Corps: 20,000–30,000 men, 2–4 divisions commanded by a major general or a lieutenant general (Confederate corps were larger than their Union counterparts.)
Army: 1+ corps, led by a commanding general. (Union armies were typically named after the rivers in the territory where they operated. Confederate armies were named from the state or regions where they were active.)
In 1863, the Union Army of the Potomac had seven infantry corps. The Confederate Army of Northern Virginia had three infantry corps. All units were officially numbered but were often also identified by their commander’s name. By 1863, many units were operating by as much as 50 percent under-strength.