British logistics played a key role in the success of Operation Overlord, the Allied invasion of France in June 1944. The objective of the campaign was to secure a lodgement on the mainland of Europe for further operations. The Allies had to land sufficient forces to overcome the initial opposition and build them up faster than the Germans could respond. Planning for this operation had begun in 1942. The Anglo-Canadian force, the 21st Army Group, consisted of the British Second Army and Canadian First Army. Between them, they had six armoured divisions (including the Polish 1st Armoured Division), ten infantry divisions, two airborne divisions, nine independent armoured brigades and two commando brigades. Logistical units included six supply unit headquarters, 25 Base Supply Depots (BSDs), 83 Detail Issue Depots (DIDs), 25 field bakeries, 14 field butcheries and 18 port detachments. The army group was supported over the beaches and through the Mulberry artificial port specially constructed for the purpose.
During the first seven weeks after the British and Canadian landings in Normandy on D-Day, 6 June 1944, the advance was much slower than anticipated, and the lodgement area was much smaller. The short lines of communication provided an opportunity to accumulate reserves of supplies. Two army roadheads were created. No. 1 Army Roadhead for I Corps and No. 2 Army Roadhead for XXX Corps, these being the two corps ashore at the time. When the Canadian First Army assumed control of the British I Corps on 21 June, the No. 1 Army Roadhead also passed into its control. No. 2 Army Roadhead formed the nucleus of what became the Rear Maintenance Area (RMA) of the 21st Army Group. By 26 July, 675,000 personnel, 150,000 vehicles and 690,000 tonnes (680,000 long tons) of stores and 69,000 tonnes (68,000 long tons) of bulk petrol had been landed. Ammunition usage was high, exceeding the daily allocation for the 25-pounder field guns by 8 per cent and for the 5.5-inch medium guns by 24 per cent. Greater priority was given to ammunition shipments, with petrol, oil and lubricant (POL) shipments cut to compensate.
On 25 July, the US First Army began Operation Cobra, the break-out from Normandy. On 26 August, the 21st Army Group issued orders for an advance to the north to capture Antwerp, Belgium. After a rapid advance, the British Guards Armoured Division liberated Brussels, the Belgian capital, on 3 September and the 11th Armoured Division captured Antwerp the following day. The advance was much faster than expected and the rapid increase in the length of the line of communications threw up logistical challenges that, together with increased German resistance, threatened to stall the Allied armies. By mid-September, the Allies had liberated most of France and Belgium. The success of the 21st Army Group was in large part due to its logistics, which provided the operational commanders with enormous capacity and tremendous flexibility.