Montgomery had been dissatisfied with the performance of the corps and its GOC, Gerard Bucknall, a fellow Middlesex Regiment officer, since the landings in Normandy two months earlier. Horrocks retained control of XXX Corps during the advance through Belgium, taking Brussels, and at one point advanced in only six days. Supplies were a constant concern; the major French deep-water ports were still in German hands, and Allied supply lines stretched back to the Normandy beaches. Montgomery's 21st Army Group was by now operating from its ports—twice the distance logistical planners had accounted for—so XXX Corps was diverted towards Antwerp to secure its docks and harbour. The city and port fell to the 11th Armoured Division in early September, but Montgomery halted XXX Corps for resupply short of the wide Albert Canal to the north of the city, which consequently remained in enemy hands. Horrocks regretted this after the war believing that his corps might have advanced another with the fuel available. Although some doubt this could have been achieved without delays, it is now known that XXX Corps was opposed by only one German division, although Allied forces were unaware of this at the time. The pause allowed the Germans to regroup around the Scheldt River, and by the time the Allies resumed their advance, the First Paratroop Army (General Kurt Student) had arrived and set up strong defensive positions along the opposite side of the canal. The task of breaking the strengthened German line, which stretched from Antwerp to the North Sea along the Scheldt River, would fall to the First Canadian Army in the month-long, costly Battle of the Scheldt. By mid-September, XXX Corps had been diverted again, this time to the east. Operation Market Garden In September, Montgomery, now a field marshal, made his ambitious thrust across the Rhine and into the German industrial heartland, codenamed Operation Market Garden, a priority for 21st Army Group. XXX Corps under Horrocks was to lead the ground assault, passing along a corridor held by airborne forces to link up with the British 1st Airborne Division in Arnhem within four days. In any event XXX Corps never arrived and although 1st Airborne clung on to their tenuous position for a further five days, by 21 September almost three-quarters of the division was destroyed or captured. Postwar analyses have been divided, some stressing a perceived lack of urgency on the part of