21st Army Group

The 21st Army Group was a World War II British headquarters formation, in command of two field armies and other supporting units, consisting primarily of the British Second Army and the First Canadian Army. Established in London during July 1943, under the command of Supreme Headquarters Allied Expeditionary Force (SHAEF), it was assigned to Operation Overlord, the Western Allied invasion of Europe, and was an important Allied force in the European Theatre. The 21st Army Group operated in Northern France, Luxembourg, Belgium, the Netherlands and Germany from June 1944 until the end of the war in Europe in May 1945, after which it was redesignated the British Army of the Rhine (BAOR).

Normandy

Commanded by General (later Field Marshal) Sir Bernard Montgomery, 21st Army Group initially controlled all ground forces in Operation Overlord (the United States First Army and British Second Army). When sufficient American forces had landed, their own 12th Army Group was activated, under General Omar Bradley, and the 21st Army Group was left with the British Second Army and the newly activated First Canadian Army which, despite its title, also contained many British and Polish troops.

Army group

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.

In the Polish Armed Forces and former Soviet Red Army an army group was known as a Front. The equivalent of an army group in the Imperial Japanese Army was a "general army" (Sō-gun (総軍)).

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 U.S. Army 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").