The Allied Invasion of Italy was the Allied landing on mainland Italy on 3 September 1943, by General Harold Alexander’s 15th Army Group (comprising Lieutenant General Mark Clark’s U.S. Fifth Army and General Bernard Montgomery’s British Eighth Army) during the Second World War. The operation followed the successful invasion of Sicily during the Italian Campaign. The main invasion force landed around Salerno on the western coast in Operation Avalanche, while two supporting operations took place in Calabria (Operation Baytown) and Taranto (Operation Slapstick). Following the defeat of the Axis Powers in North Africa, there was disagreement between the Allies as to what the next step should be. Winston Churchill in particular wanted to invade Italy, which in November 1942 he called “the soft underbelly of the axis” (and General Mark Clark later called “one tough gut”). Popular support in Italy for the war was declining, and he believed an invasion would remove Italy, and thus the influence of axis forces in the Mediterranean Sea, opening it to Allied traffic. This would very materially reduce the amount of scarce shipping capacity needed to supply Allied forces in the Middle East and Far East at a time when the disposal of Allied shipping capacity was in crisis and increase British and American supplies to the Soviet Union. In addition, it would tie down German forces, keeping them away from the Russian front. Stalin had been pressing to open a “second front” in Europe, which would weaken the Wehrmacht’s invasion of Russia.
However, General George Marshall and much of the American staff wanted to avoid operations that might delay an invasion of Europe, discussed and planned as early as 1942, which finally materialized as Operation Overlord. When it became clear that no invasion could be undertaken in 1943, it was agreed to invade Sicily, with no commitment made to any follow-up operations. However, both Roosevelt and Churchill accepted the necessity of Allied armies continuing to engage the Axis in the period after a successful campaign in Sicily and before the start of one in northwest Europe  The discussion continued through the Trident Conference in Washington in May but it was not until late July, after the course of the Sicily campaign had become clear and with the fall of Mussolini, that the Joint Chiefs of Staff instructed Eisenhower to go ahead at the earliest possible date
Joint Allied Forces Headquarters AFHQ were operationally responsible for all Allied land forces in the Mediterranean theatre, and it was they who planned and commanded the invasion of Sicily and the Italian mainland.
The Allied invasion of Sicily in July 1943, codenamed Operation Husky, was highly successful, although many of the Axis forces managed to avoid capture and escape to the mainland. To the Axis, this was viewed as a success. More importantly in late July a coup deposed Benito Mussolini as head of the Italian government, which then began approaching the Allies to make peace. It was believed a quick invasion of Italy might hasten an Italian surrender and produce quick military victories over the German troops that could be trapped fighting in a hostile country. However, Italian (and more so German) resistance proved relatively strong, and fighting in Italy continued even after the fall of Berlin. In addition, the invasion left the Allies in a position of supplying food and supplies to conquered territory, a burden which would otherwise have fallen on Germany. As well, Italy occupied by a hostile German army would have created additional problems for the German Commander-in-Chief Albert Kesselring.
Prior to Sicily, Allied plans envisioned crossing the Strait of Messina, a limited invasion in the “instep” area (Taranto), and advancing up the toe of Italy, anticipating a defense by both German and Italian forces. The overthrow of Benito Mussolini and the Fascisti made a more ambitious plan feasible, and the Allies decided to supplement the crossing of the Eighth Army with a seizure of the port of Naples. They had a choice of two landing areas: one at the Volturno River basin and the other at Salerno, both at the range limits of Allied fighter planes based in Sicily. Salerno was chosen because it was closer to air bases, experienced better surf conditions for landing, allowed transport ships to anchor closer to the beaches, had narrower beaches for the rapid construction of exit roads, and had an excellent pre-existing road network behind the beaches.
Operation Baytown was the preliminary step in the plan in which Eighth Army, under General Bernard Montgomery would depart from the port of Messina on Sicily, to cross the Straits of Messina and land near the tip of Calabria (the “toe” of Italy), on 3 September 1943. The short distance from Sicily meant landing craft could launch from there directly, rather than be carried by ship. V British Corps’ 5th Infantry Division would land on the north side of the “toe” while its 1st Canadian Infantry Division would land at Cape Spartivento on the south side. General Montgomery was strongly opposed to Operation Baytown. He predicted it would be a waste of effort since it assumed the Germans would give battle in Calabria; if they failed to do so, the diversion would not work, and the only effect of the operation would be to place the Eighth Army 300 mi (480 km) south of the main landing at Salerno. He was proved correct; after Operation Baytown the Eighth Army marched 300 miles north to the Salerno area against no opposition other than engineer obstacles.
Plans for the use of airborne forces took several forms, all of which were cancelled. The initial plan to land glider-borne troops in the mountain passes of the Sorrento Peninsula above Salerno was abandoned 12 August. Six days later it was replaced by Operation Giant, in which two regiments of the 82nd U.S. Airborne Division would seize and hold crossings over the Volturno River. This was at first expanded to include the entire division, including an amphibious landing by the glider regiment, then deemed logistically unsupportable and reduced to a two-battalion drop at Capua to block the highway there. The Italian surrender on 3 September cancelled Operation Giant I and replaced it with Operation Giant II, a drop of the 504th Parachute Infantry Regiment on Stazione di Furbara and Cerveteri airfields, 25 mi (40 km) northwest of Rome, to aid Italian forces in saving Rome from the Germans, a condition of the Italian armistice. Because the distance from the Allied beachheads precluded any substantial Allied support of the airborne troops, Brig. Gen. Maxwell D. Taylor, 82nd Airborne’s Assistant Division Commander, was spirited into Rome to assess the willingness of Italian troops to cooperate with the Americans. Taylor’s judgment was the operation would be a trap and he advised cancellation, which occurred late on the afternoon of 8 September as troop carriers were preparing to take off.
The main landings (Operation Avalanche) were scheduled to take on 9 September, during which the main force would land around Salerno on the western coast. It would consist of the U.S. Fifth Army under Lieutenant General Mark W. Clark, comprising the U.S. VI Corps under Major General Ernest J. Dawley, the British X Corps under Lieutenant General Richard McCreery, with 82nd Airborne in reserve, a total of eight divisions and two brigade-sized units. Its primary objectives were to seize the port of Naples to ensure resupply, and to cut across to the east coast, trapping Axis troops further south. The naval task force of warships, merchant ships and landing craft totaling 627 vessels came under the command of Vice Admiral Kent Hewitt. Part of Hewitt’s command was Force V which included five aircraft carriers to provide air cover for the landings. Cover for the task force was provided by Force H, a group of four British battleships and two fleet carriers with destroyers in support, which was directly subordinate to the C–in–C Mediterranean Admiral of the Fleet Sir Andrew Cunningham.
In the original planning, the great attraction of capturing the important port of Taranto in the “heel” of Italy had been evident and an assault had been considered but rejected because of the very strong defenses there. However, with the signing of the armistice with the Italians on 3 September the picture changed. It was decided to carry the British 1st Airborne Division to Taranto using British warships, seize the port and several nearby airfields and follow up by shipping in British V Corps and a number of fighter squadrons. The airborne division, which was undergoing training exercises in two locations 400 mi (640 km) apart, was ordered on 4 September to embark on 8 September. With such short notice to create plans, Operation Slapstick was soon nicknamed Operation Bedlam
The Avalanche plan was daring but flawed; Fifth Army would be landing on a very broad 35 mi (56 km) front, using only three assault divisions (two British in X Corps, one American in VI Corps), and the two Corps were widely-separated both in distance (12 mi (19 km) and by the Sele River. Clark initially provided no troops to cover the river, offering the Germans an easy route to attack, and only belatedly landed two battalions to protect it. Furthermore, the terrain was highly favorable to the defender. Planning for the Salerno phase was accomplished in only forty-five days, rather than the months that might be expected.A U.S. Army Ranger force under Colonel William O. Darby consisting of three U.S. Ranger battalions and two British Commando units was tasked with holding the mountain passes leading to Naples, but no plan existed for linking the Ranger force up with X Corps’ follow-up units. Finally, although tactical surprise was unlikely, Clark ordered no naval preparatory bombardment or naval gunfire support take place, despite experience in the Pacific Theatre demonstrating it was necessary. (Major General Fred Walker, commanding 36th “Arrowhead” Division, believed the defenders, from LXXVI Panzer Corps, were too scattered for it to be effective.)
On the German side, Albrecht von Kesselring lacked the strength to push the Salerno landing back, and was refused two panzer divisions from northern Italy to assist him.
Operation Avalanche was planned under the name Top Hat and supported by a deception plan, Operation Boardman, a false threat of an Allied invasion of the Balkans.
Axis defensive organization.
In mid-August, the Germans had activated Army Group B (Heeresgruppe B) under Erwin Rommel with responsibility for German troops in Italy as far south as Pisa. Army Command South (OB Süd) under Albert Kesselring continued to be responsible for southern Italy and the German High Command formed a new army headquarters to be Army Command South’s main field formation. The new Tenth Army (10. Armee) headquarters, commanded by Heinrich von Vietinghoff, was activated on 22 August. German Tenth Army had two subordinate corps with a total of six divisions which were positioned to cover possible landing sites. Under XIV Panzer Corps (XIV Panzerkorps) was Hermann Göring Panzer Division (Fallschirm-Panzer Division 1 Hermann Göring), 15th Panzergrenadier Division (Panzergrenadier-Division) and 16th Panzer Division (16. Panzer-Division); and under LXXVI Panzer Corps (LXXVI Panzerkorps) was 26th Panzer Division (26. Panzer-Division), 29th Panzergrenadier Division (29. Panzergrenadier-Division) and 1st Parachute Division (1. Fallschirmjäger-Division). von Vietinghoff specifically positioned the 16th Panzer Division in the hills above the Salerno plain.