Military formation (ground)

A formation in military terms is a grouping of units.

Brigade

A brigade is the smallest type of [combined]]formation, and generally consists of two or more battalions plus headquarters troops.

Division

Military divisions date back to the Napoleonic era. By the time of World War I, a division generally consisted of two or more regiments/brigades and numbered between 10 and 20,000 soldiers. It is usually commanded by a major general.

Corps

A corps is a unit of two or more divisions; usage of this term dates back to before the American Civil War. Corps are usually commanded by lieutenant generals, although corps that are not independent of a field army may be commanded by a major general.

In some armies, a corps is geographic rather than tactical, or may represent a branch of specialization.

Army

An Army is a grouping of, usually, two or more corps. Armies may also be responsible for the zone of communications behind a major front. They can also be repositories for "army-level" assets, such as super-heavy artillery, heavy armor, or other units of a specialized type not appropriately controlled by lower level headquarters.

An Army is generally commanded by a General or equivalent.