As Supreme Commander, the American General Dwight Eisenhower was responsible for the invasion of Sicily, codenamed Operation Husky. Direct control of the forces on the ground, however, fell to the British General, Harold Alexander, whose 15th Army Group consisted of General Bernard Montgomery's 8th British Army, and General George Patton's 7th US Army.

The ultimate aim of the invasion was to seize the ports of Palermo and Messina, on the northern-western and north-eastern sides of the island respectively. These cities, particularly Messina, were the only escape routes for the Axis forces from Sicily, and the Allies hoped to trap and take prisoner as many of them as possible.

Montgomery felt that the invasion should be under the direction of a single general, fearing that too many cooks might spoil the broth. Naturally, this general would have been himself, and the 8th Army alone would invade Sicily with the II US Corps placed under its command. Montgomery knew that it would offend American ideology to place their troops under the control of a foreign general, so he was not at all surprised that the proposal came to nothing and that the 7th US Army was granted an independent role in the invasion, under the vague auspices of General Alexander's 15th Army Group.

While the fighting in North Africa was still in progress, eight successive proposals for Operation Husky had been outlined by Alexander's staff, but Montgomery had rejected them all as unrealistic. The recurrent theme was that the 7th US Army should land in the north, close to Palermo, and the 8th British Army along the eastern and southern coastline to the south of Messina. This was effectively to say that the armies would be put ashore on opposite sides of a 150 mile wide island, which was too great a distance for them to have any hope of coordinating their efforts. Furthermore the component parts of both armies were detailed to land on multiple beaches across a wide front. Such a dispersal of strength would work well if enemy resistance was minimal, but Montgomery expected it to be strong and he therefore favoured concentrating the 8th Army into a tighter beachhead so that its divisions would be better placed to lend support to each other in the event of difficulties. He also, quite correctly, asserted that the proposed landings at Palermo and Messina were unworkable as both areas were out of range of Allied fighter aircraft based in North Africa, and as such enemy air attacks would hinder every endeavour of the ground forces and restrict the ability of the Navy to keep them supplied. The British had learned earlier in the war happens to armies when the skies above them are dominated by enemy aircraft.

Montgomery had a great deal of respect for General Alexander, but both he and Patton felt that he was something of a "fence-sitter", and he also believed that the staff of the 15th Army Group lacked sufficient experience to be capable of assembling a feasible plan. Consequently Montgomery took the initiative and, writing to Alexander on the 24th April, detailed his objections and outlined his own proposal, ultimately accepted, for the conduct of the invasion. He said, "Planning to date has been on the assumption that resistance will be slight and Sicily will be captured easily. Never was there a greater error. Germans and Italians are fighting well in Tunisia and will repeat the process in Sicily. If we work on the assumption of little resistance, and disperse our effort as is being done in all planning to date, we will merely have a disaster. We must plan for fierce resistance, by the Germans at any rate, and for a real dog fight battle to follow the initial assault."

The final plan called for both armies to land in the south of Sicily, within the range of friendly fighter aircraft. The 7th US Army was to concentrate along a 20 mile front between Licata and Gela, in the centre of the coastline, while the 8th Army landed some 50 miles to the east, along a 35 mile front between Syracuse and Pachino, at the south-eastern tip of the island. Although this particular area was not strategically significant, Montgomery had chosen it as a suitable firm base from which to advance inland, and it was also within immediate grabbing distance of key enemy air and naval bases. Securing airfields to enable Allied fighters to act effectively over Sicily was vital to the plan, and it was with this in mind that the Americans were to focus their initial efforts on the capture of Gela.

The plan thereafter was not firmly established. General Alexander had no definite idea of how the campaign would proceed from the beaches, which had the result that the 7th and 8th Armies were effectively allowed to devise their own plans with little thought of coordinating their efforts to achieve optimum effect. This lack of effective control on behalf of the 15th Army Group had serious repercussions for the conduct of the operation.

Montgomery's broad intention, although he had no authority to commit anyone else to it, was for the 8th Army to pin down the main enemy forces by advancing north along the short but heavily fortified eastern coastal road, while the Americans moved rapidly through the relatively sparsely defended terrain along their left flank. By reaching the north coast first, the 7th Army would cut Sicily in two and trap all Axis forces to the west. They could then proceed with all haste to the north-eastern tip of the island and capture Messina; the only escape route for the Axis troops in front of the 8th Army. In this way it was hoped that the campaign would be brought to a swift conclusion and another great haul of prisoners would fall into Allied hands. Achieving this final point was crucial for the expansion of the war in the Mediterranean; if substantial enemy forces could be trapped in Sicily, mainland Italy would be ripe for the taking.

Yet before all of these wonderful possibilities could be realised, the immediate difficulty confronting the Allies was the fact that even a dull-witted enemy would need only half a glance at a map to realise that Sicily, dominating the central Mediterranean as it did, would be a painfully obvious next step for the Allies to take. An invasion force of any magnitude is extremely vulnerable to counterattack during the first days of its landing, lest it is thrown back into the sea before a firm foothold can be established. In answer to this problem, the British, who had long since developed a particular skill for feeding their opponents perfectly plausible deceptions, put Operation Mincemeat into effect, and in so doing pulled off one of their most spectacular successes of the war.

During the early hours of the 30th April 1943, a Royal Navy submarine surfaced off the Spanish coast and pushed a dead body, in military uniform, into the sea. The true identity of this man has never been established, but British Intelligence had named him Major William Martin of the Royal Marines, and his body was made to appear as though he had died as a result of an air crash. Various devices were used to make his identity and death appear plausible, all in turn to lend credibility to the falsified secret documents carried on his person. These cleverly acknowledged how obvious an objective Sicily would be by suggesting that the Allies would indeed invade it, but only as a diversion for the main landings that were to take place in Sardinia and Southern Greece shortly after.

The Spanish discovered "Major Martin's" body, and although they were a neutral power in the war, their fascist government naturally had pro-German sympathies and, as the British had hoped, Berlin was immediately informed of the find. Germany had expected that Sicily would be the next Allied objective, and although a few of their commanders remained suspicious of this new information and still looked to the island with great concern, Hitler and many others were convinced. Accordingly, a significant part of Germany's reserve was dispatched to Sardinia and Greece, whilst only two divisions were left alongside the Italians in the defence of Sicily. During May, Winston Churchill was informed, "Mincemeat swallowed whole".

The invasion was scheduled to take place before dawn on the 10th July, and it was to be spearheaded by the British 1st Airborne Division and the American 82nd Airborne Division. Elements of these elite units were to drop at night, several hours ahead of the seaborne landings, to capture or destroy a number of vital objectives in their respective invasion areas. Their success would make the landing of the Allied armies a great deal more straightforward and lay the foundations for their initial thrusts inland.