Rommel attempted to turn the left side of the Eighth Army at the Battle of Alam Halfa from 31 August 1942. The German/Italian armoured Corps infantry attack was stopped in very heavy fighting. Rommel's forces had to withdraw quickly so that they could escape though the British minefields be cut off. Montgomery was criticised for not counter-attacking the retreating forces immediately, but he felt strongly that his build-up of British forces was not yet ready. A hasty counter-attack risked ruining his strategy for an offensive on his own terms in late October, planning for which had begun soon after he took command. He was confirmed in the permanent rank of lieutenant-general in mid October.
The conquest of Libya was essential for airfields to support Malta and to threaten the rear of Axis forces opposing Operation Torch. Montgomery prepared meticulously for the new offensive after convincing Churchill that the time was not being wasted. (Churchill sent a telegram to Alexander on 23 September 1942 which began, "We are in your hands and of course a victorious battle makes amends for much delay.) He was determined not to fight until he thought there had been enough preparation for a victory, and put into action his beliefs with the gathering of resources, detailed planning, the training of troops — especially in clearing minefields and fighting at night — and in the use of 252 of the latest American-built Sherman tanks, 90 M7 Priest self-propelled howitzers, and making a personal visit to every unit involved in the offensive. By the time the offensive was ready in late October, Eighth Army had 231,000 men on its ration strength including British, Australian, South African, Indian, New Zealand, Greek and Free French units.
The Second Battle of El Alamein began on 23 October 1942, and ended twelve days later with the first large-scale, decisive Allied land victory of the war. Montgomery correctly predicted both the length of the battle and the number of casualties (13,500). However, soon after British armoured units and infantry broke through the German and Italian lines and were pursuing the enemy forces at speed along the coast road, a violent rainstorm burst over the region, bogging down the tanks and support trucks in the desert mud. Montgomery, standing before his officers at headquarters and close to tears, announced that he was forced to call off the pursuit. Corelli Barnett has pointed out that the rain also fell on the Germans, and that the weather is therefore an inadequate explanation for the failure to exploit the breakthrough, but nevertheless the Battle of El Alamein had been a great success. Over 30,000 prisoners were taken including the German second in command, General von Thoma, as well as eight other general officers. Rommel, having been in a hospital in Germany at the start of the battle, was forced to return on 25 October 1942 after General Stumme - his replacement as German commander - died of a heart attack in the early hours of the battle.
Montgomery was knighted and promoted to full general. The Eighth Army's later advance as the Germans retreated hundreds of miles towards their bases in Tunisia used the logistical as well as the firepower advantages of the British Army while avoiding unnecessary risks. It also gave the Allies an indication that the tide of war had genuinely turned in North Africa . Montgomery kept the initiative, applying superior strength when it suited him, forcing Rommel out of each successive defensive position. On 6 March 1943, Rommel's attack on the over-extended Eighth Army at Medenine (Operation Capri) with the largest concentration of German armour in North Africa was successfully repulsed. At the Mareth Line, 20 March to 27 March, when Montgomery encountered stronger opposition than he had expected, he switched to trying to move around the side of the Germans, backed by low-flying RAF fighter-bomber support.
This campaign demonstrated the battle-winning ingredients of morale (sickness and absenteeism were virtually eliminated in the Eighth Army), co-operation of all arms including the air forces, first-class logistical back-up and clear-cut orders. For his role in North Africa he was awarded the Legion of Merit by the United States government in the rank of Chief Commander.
The next major Allied attack was the Allied invasion of Sicily (Operation Husky). Montgomery considered the initial plans for the Allied invasion, which had been agreed in principle by Eisenhower and Alexander, to be unworkable because of the way troops and effort were separated. He managed to have the plans changed to concentrate the Allied forces, having Patton's Seventh US Army land in the Gulf of Gela (on the left side of Eighth Army, which landed around Syracuse in the south-east of Sicily) rather than near Palermo in the west and north of Sicily. Inter-Allied tensions grew as the American commanders Patton and Bradley (then commanding II US Corps under Patton), got annoyed because they saw Montgomery as boastful. They resented him, while accepting his skills as a general.
During the autumn of 1943, Montgomery continued to command the Eighth Army during the landings on the mainland of Italy itself. In conjunction with the Anglo-American landings at Salerno (near Naples) by Mark Clark's Fifth Army and seaborne landings by British paratroops in the heel of Italy (including the key port of Taranto, where they disembarked without resistance directly into the port), Montgomery led the Eighth Army up the toe of Italy. Some criticism was made of the slowness of Montgomery's advance. The Eighth Army, responsible for the eastern side of the Allied front, from the central Apennine mountain spine to the Adriatic coast, fought a succession of engagements alternating between opposed crossings of the rivers running across their line of advance and attacks against the cleverly constructed defensive positions the Germans had fashioned on the ridges in between. The Eighth Army crossed the Sangro river in mid-November and penetrated the German's strongest position at the Gustav Line but as the winter weather deteriorated the advance ground to a halt as transport bogged down and air support operations became impossible. Montgomery hated the lack of coordination, the dispersion of effort, and the strategic muddle and opportunism he perceived in the Allied effort in Italy and was glad to leave the "dog's breakfast" on 23 December
Montgomery returned to Britain to take command of the 21st Army Group which consisted of all Allied ground forces that would take part in Operation Overlord, the invasion of Normandy. Preliminary planning for the invasion had been taking place for two years, most recently by COSSAC staff (Chief of Staff to the Supreme Allied Commander).
Montgomery's initial plan was, most likely, for an immediate breakout toward Caen. He did not have enough men at first, so started a series of battle in which the British, Canadian and American armies trapped and defeated the German forces in Normandy in the Falaise pocket. By the middle of July the Cotentin Peninsula was occupied and Caen captured.
The increasing number of American troops in the European theatre (from five out of ten divisions at D-Day to 72 out of 85 in 1945) made it a political impossibility for the Ground Forces Commander to be British. After the end of the Normandy campaign, General Eisenhower himself took over Ground Forces Command while continuing as Supreme Commander, with Montgomery continuing to command the 21st Army Group, now consisting mainly of British and Canadian units. Montgomery bitterly resented this change, although it had been agreed before the D-Day invasion. Winston Churchill had Montgomery promoted to Field Marshal by way of compensation.