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. At various times during its existence, the 21st Army Group had additional British, Canadian, American and Polish field armies or corps attached to it. The 21st Army Group operated in Northern France, Luxembourg, Belgium, the Netherlands and Germany from June 1944 until August 1945, when it was renamed the British Army of the Rhine (BAOR).