An army group is a military organization consisting of several field armies, which is self-sufficient for indefinite periods. It is usually responsible for a particular geographic area. An army group is the largest field organization handled by a single commander—usually a full general or field marshal—and it generally includes between 400,000 and 1,000,000 soldiers.
Army groups may be multi-national formations. For example, during World War II, the Southern Group of Armies (also known as the U.S. 6th Army Group) comprised the U.S. Seventh Army and the French First Army; the 21st Army Group comprised the British Second Army, the Canadian First Army and the US Ninth Army.
In both Commonwealth and U.S. usage, the number of an army group is expressed in Arabic numerals (e.g., "12th Army Group"), while the number of a field army is spelled out (e.g., "Third Army").