Article describes the levels of organization of northern and southern armies and the officers who commanded at each level.

To the non-military buff, the organization and terminology used for Civil War armies can be very confusing. The "Army of the Potomac" was the main Union army in the eastern theater of the war and the "Army of Northern Virginia" was the main Confederate force. Remarkably, both of the armies that fought the Battle of Gettysburg were organized in a similar fashion including a structure of corps, divisions, and brigades. But what were these different organizations and how did they all fit in to one huge force?

The Federal government and the Confederate government both had war departments, which oversaw the organization, supply, and movements of their respective armies. Civil War-era armies were organized according to military manuals including those adopted by the Federal War Department prior to 1861. Because the war had to be fought over a large area of the South, the Union and the Confederacy both had several armies, each fighting in different "theaters" or sections of the country. Each army was a structured organization that included a general headquarters, infantry, dominionartillery, cavalry, signalmen, engineers, quartermaster and commissary departments. The largest single organization of an army was a corps (pronounced "core"). The Union Army at Gettysburg had seven infantry corps and a cavalry corps, each commanded by a major general. The Confederate Army had three infantry corps, each commanded by a lieutenant general. Typically, a Confederate corps was much larger than a Union corps. A corps included three infantry divisions and an artillery brigade in the Union army or an artillery battalion in the Confederate Army. The Army of the Potomac had distinguishing symbols called corps badges to signify one corps from another. The badges were actually small cloth cut-outs shaped like crosses, spheres, stars, and quarter moons, and made in three different colors- red, white, and blue, each color specific to a division of the corps. Confederates had no corps badges or particular symbols for their organizations.

The infantry division was commanded by a major or a brigadier general and composed of two to four infantry brigades. The brigade, commanded by a brigadier general, was composed of four to six regiments, and was the primary organization used by commanders in battle. A brigade with good officers and good training was a formidable fighting force and often advanced or defended positions in cooperation with fellow brigades. It was common practice for a brigade commander to send forward most of his regiments and hold one in reserve. The Confederate War Department made attempts to have brigades composed of regiments from one singular state or state affiliation, such as General Joseph Kershaw's brigade composed of all South Carolina regiments. The Union Army did not always make such conscious choices, though there were some brigades which acquired interesting nick names due to their ethnic origin or locality from which they hailed.

For the infantryman, the regiment was the most important unit. Led by a colonel, lt. colonel and major, a full strength regiment numbered over 1,000 officers and men. Attrition due to disease and battle losses meant considerably lower personnel in each regiment by the time of the Battle of Gettysburg, where some regiments mustered only about two-hundred. A regiment was divided into ten companies of 100 men each at full strength. One company was divided in half as two platoons. One company was led by a captain with two lieutenants who each commanded a platoon. Platoons were divided into squads, led by a sergeant or corporal. Regiments fought in a "battle line" or in some cases a "skirmish line", which was a general open rank tactic used to feel out the strength of an enemy force.

War Departments in the North and South issued regulations for how the army was to be organized in the field though commanders were also given the luxury of making changes in the organization as they saw fit. Thus there were often variances from the chart seen here especially in the Army of the Potomac, which lost regiments due to the expiration of their term of service or consolidated organizations due to battle-related attrition. Though the Union army's corps were each designated to have three divisions, the Third Corps and Twelfth Corps only had two divisions apiece at Gettysburg. Likewise, the Eleventh Corps' organization varied from the regulations having only two brigades in each of its three divisions.

At the outbreak of the Civil War, there was a standing force of "regular" units in the United States Army. State militias were called into service, but there was a need to Federalize these units so that they could muster pay from the United States government and serve outside of state borders. Each state was given a quota of "volunteer regiments" to be raised for service lasting from three months to three years. The South faced a similar dilemma. Southern states raised and supplied the Confederate armies with volunteer regiments. By 1863, many of the regiments in both armies had been in service since 1861 and were still composed of mostly volunteer soldiers, though the first "conscripts" or men required by state law to serve in the military defense of a state, had begun to appear in Southern units. A regiment's flag, or "regimental colors", were painted with the regiment's number and state affiliation, usually followed by "VOLUNTEER INFANTRY." The term volunteer was a symbol of pride for soldiers on both sides.

The most widely used manual for small units (regiments) was Rifle and Light Infantry Tactics For The Exercise and Maneuvers of Troops When Acting As Light Infantry Or Riflemen, written by William J. Hardee. The manual specified the proper placement of officers, the rank and file, the manual of arms, basic marching orders, and other requirements. His manual was re-written for Confederate use in 1861 when Hardee resigned his commission from the United States Army and joined the Confederacy. Other manuals of organization and drill were used, but "Hardee's Tactics" continued to be the most popular and widely used manual throughout the war.

The artillery was usually organized by regiments as well, except that each company was called a battery. A battery consisted of over 100 soldiers, armed with six cannon per battery. Confederate batteries were smaller, some having only four cannon. Batteries were assigned independently from their regiments to specific artillery brigades (Union) or battalions (Confederate) or to the artillery reserve of an army. Both of the armies had an artillery reserve which was an organization of extra batteries to be placed where needed. The Union army had one large artillery reserve force. The Confederate army had one reserve group per corps, but the number of guns was still smaller than the number of Union cannon.

A cavalry regiment was organized in a similar fashion to the infantry and artillery. Ten to twelve companies or "troops", made up one regiment. The regiment was divided into three battalions, each composed of four companies. A company was divided into "squadrons" for easy maneuvering on the field. The cavalry regiment was much more expensive to sustain while in service due to the amount of equipment carried by each cavalryman (carbine, saber, pistol, belt set, and equipment for the soldier's mount) and the requirement for horses and their care.

Both armies also had a compliment of quartermaster, engineer, and signal units as well as supply wagons organized as "trains". An army on the march was usually followed by miles and miles of wagons loaded with the equipments of war including food, ammunition, and medical supplies. At the top of the organizational list was the Army Headquarters. The commanding general required a personal staff to dictate orders and keep records of army movement. There were also clerks and assistants. The commanders of armies also had the privilege of a headquarters cook. Every army headquarters usually had a large compliment of staff officers, couriers, and a headquarters guard, which included an infantry battalion and a cavalry escort.