It was activated in 1943 in Algiers, North Africa, to plan the invasion of Sicily. Its main forces for this job were the British Eighth Army and the US Fifth Army. For some time, it was under command of British Field MarshalHarold Alexander. By mid-1944, though, the small-scale invasion that had been planned earlier blew into a theatre of its own. 15th Army Group had pushed northward through Italy, capturing Rome, and driving the retreating Axis forces into Northern Italy. In December, American Lieutenant GeneralMark W. Clark became the new commander, and by 1945, they had an unparalleled superiority in Central Europe.
An armygroup is a military organization (formation) consisting of several field armies, and is supposed to be self-sufficient for indefinite periods.
An ArmyGroup is the largest field organization handled by a single commander in modern warfare, and usually includes between 400,000 and 1,500,000 troops.
For example in 1944–45 during the Second World War, the Southern Group of Armies (U.S. 6th ArmyGroup) was comprised of the US Seventh Army and the French First Army; and Montgomery's 21st ArmyGroup was comprised of the British Second Army, the Canadian First Army, and the US Ninth Army.