The two armies that met in the Battle of Gettysburg – the North’s Army of the Potomac and the South’s Army of Northern Virginia – shared the same heritage, traditions, and structure. Many of their senior officers had gone to the same classes and even been roommates at the United States Military Academy. They had fought side by side in the Mexican War and on the frontier. Most of the manpower for the armies had been raised by the states as volunteer regiments and batteries following similar procedures. It is no surprise that both armies had a similar organization.
The basic building block of both armies was the regiment, recruited by the individual states under a colonel and officers appointed by the state’s governor. These usually started out with a strength of 1,000 men in ten companies but rarely made it into the field with anywhere near that number. Once in the field disease and casualties continued to whittle down a regiment’s size while replacements were rare (for Confederates) or nonexistent (for the Union). The result was that infantry regiments at Gettysburg averaged around 300 to 350 men.
Brigades would group together anywhere from three to six regiments. Some of the veteran Union brigades who had seen heavy fighting would add a new regiment or two to make up for the reduced size of veteran units. Union brigades would almost always be made up of regiments from more than one state, while it was Confederate policy (aggressively pushed by President Davis) to brigade together units from the same state under a commander from that state. Brigades were nominally commanded by brigadier generals, but battle casualties or other circumstances would often result in the brigade being commanded by its senior regimental officer, usually a colonel.
Divisions were made up of two or more brigades. The North had 19 infantry divisions at Gettysburg, the South had 8. But Southern divisions were bigger; the smallest Southern division (Pickett’s, with only 3 brigades) had more men than the strongest Northern divisions, and the strongest Southern division (Rodes’) was twice as strong as all but 6 of the Northern Divisions. Divisions were nominally commanded by major generals, but at Gettysburg 14 Northern divisions were commanded by brigadier generals at the start of the battle.
Corps were made up of two or more divisions. Just like with divisions, there were less Southern corps (3 vs. 7) but they were much larger, averaging 21,000 men each vs. around 11,000 for the North. Southern corps commanders held the rank of lieutenant general, which would not be re-created in the North until 1864, and then solely for Ulysses Grant. All Northern corps commanders were major generals.
The North also had a corps of 3 divisions of cavalry, vs. the South’s single division of cavalry. Despite the apparent difference in structure, the North had less than twice as much cavalry at Gettysburg as the South.
A greater difference between the two armies was in the organization of their artillery. Lee’s artillery batteries were organized into battalions. One artillery battalion was assigned to each division and two additional battalions were assigned to each corps. The artillery of Meade’s army was organized into brigades with one assigned to each infantry corps and two brigades to the cavalry corps. In addition, five brigades of artillery formed an artillery reserve whose batteries could be attached to any unit as needed or grouped for special efforts in attack or defense.