The North African campaign ended on 13 May 1943 with the defeat of the Axis forces.
Between 14 and 24 January 1943, the Allies had met in Casablanca to decide what steps to take after the expected victory in North Africa.

During the conference, the British and US generals had strong disagreements over the war's strategic priorities against the Axis: the US generals wanted to focus all efforts on landing in northern France, while the British wanted to invade Italy at a time when preparations for the invasion were underway at home, partly due to Britain's traditional interests in the Mediterranean.
The key agreed points of the final document were: Roosevelt's declaration of unconditional surrender to be imposed on the enemy; operational decisions regarding the war in the Mediterranean; and the programme of strategic bombing of German territory.
The section of the document relating to the planning of the Italian campaign set out three objectives for the occupation of Sicily: securing the Mediterranean communication route for the Allied forces, relieving pressure on the Soviet army from the German army on the Eastern Front and intensifying pressure on Italy — which Winston Churchill referred to as the 'soft underbelly' of Europe — to force it out of the conflict.

At the Trident Conference in Washington (12–25 May 1943) and the Quadrant Conference in Quebec (17–24 August 1943), the Allied Joint Chiefs of Staff agreed on the most important operations in Europe for 1944. Operation Overlord, the Allied landing in northern France, was prioritised in terms of human resources and equipment, while operations in the Mediterranean aimed to immobilise as many German divisions as possible to facilitate Overlord's success.
Other desirable results could also be pursued, such as bringing Italy out of the war and exposing Germany's southern border to Allied forces advancing from Italy. Possessing airfields in Italy would allow Allied bombers to attack industrial areas in southern Germany and south-eastern Europe.
The surrender of Italy and the defection of Italian occupation troops in southern France and the Balkans would force Germany to send its own military forces to those territories. Occupying Sardinia and Corsica would allow the Allies to control the western Mediterranean and pave the way for an invasion of southern France.

On 10 July 1943, the Allies, under the command of General Harold Alexander, landed in Sicily (Operation Husky), beginning a slow and bloody advance towards the Alps.
The Germans had prepared plans for the event of their Italian ally's defection (Plan Achse), and General Erwin Rommel presented Hitler with two strategies for such a case: his own, which involved withdrawing German troops to the Alps and creating a strong line of defence on Germany's southern borders; and that of General Albert Kesselring, which involved a slow retreat northwards, forcing the enemy to advance inch by inch and inflicting heavy losses.
Ultimately, Kesselring's approach was adopted, resulting in numerous bloody battles and senseless artillery and aerial bombardments, causing significant casualties among both military personnel and civilians.

On 3 September 1943, the British 8th Army, commanded by General Bernard Montgomery, landed in Calabria, setting foot on mainland Italy.

On 8 September 1943, Italy signed an armistice with the Allies in Cassibile, effectively unconditionally surrendering. This was followed by the Additional Conditions of the Armistice with Italy, which were signed on 23 September in Malta.
Meanwhile, the German army rushed to occupy Italy and disarm the armed forces.
At dawn on 9 September, King Victor Emmanuel III, Marshal of Italy Pietro Badoglio (the head of the government) and some members of the royal household and military leadership hastily abandoned Rome for Brindisi.
The armed forces and state apparatus were left without orders or instructions on how to deal with the consequences of the armistice. Entire divisions were disarmed by the Germans and many soldiers were interned or killed, as in the massacre on the island of Kefalonia between 23 and 28 September 1943.

On 9 September 1943, the US Fifth Army, commanded by General Mark Clark, landed in Salerno. It entered Naples on 5 October. After a slow approach to the main German defensive lines on the Gustav Line from 12 October to 15 November due to fierce enemy resistance, the bloody battles of Cassino began in mid-January 1944 and lasted until the end of May, resulting in heavy losses for the Allied troops.
In an attempt to break the deadlock, the VI Army Corps landed at Anzio on 22 January 1944 under the command of General John Lucas. The aim was to quickly advance towards Rome and cut off the supply and retreat lines of the German forces stationed on the Gustav Line.

Notably, on 13 October 1943, the Badoglio government declared war on Germany, obtaining co-belligerent status from the Allies.

Between 28 November and 1 December 1943, Franklin D. Roosevelt, Winston Churchill and Joseph Stalin met in Tehran to define the war strategy for the immediate future and the reorganisation of Europe after the end of the conflict.

The bloody battle on the Gustav Line, near Cassino, held back the Allied armies from 12 January to 18 May 1944. Once this line of defence was broken, the advance from the beaches of Anzio and Nettuno also began.
Contrary to his orders, General Mark Clark entered Rome on 4 June 1944, two days before Operation Overlord — the Allied invasion of northern France — began. From that moment on, the Italian front became less important.
The Italian front moved to third place in the Allied priorities in the European theatre, preceded by the landing in southern France on 15 August 1944 in Provence, which was called Operation Anvil-Dragoon. Roosevelt had insisted on this operation and essentially imposed it on Churchill, who was opposed to diverting forces and resources from the Italian front.
As a result, the US 5th Army lost seven divisions, a group of bombers and 23 fighter squadrons as it prepared to attack the powerful German defensive line in the Apennines, known as the Gothic Line.
The 5th Army was reinforced by the 92nd US Infantry Division in September and the 1st Brazilian Infantry Division in October, while the 10th Mountain Division was preparing to leave America.
Meanwhile, on the German front, Kesselring received eight new divisions, including the Hermann Göring Panzer Division.

By the second half of 1944, the spectre of defeat that had hung over Germany since the Battle of Stalingrad (19–2 February 1943) had become a certainty. The German army was on the defensive on all fronts. On the eastern front, the Soviet army dealt a severe blow to the Wehrmacht in the Battle of Kursk between 5 July and 23 August 1944 and advanced towards the German borders.
On the western front, the Allied divisions advanced towards the German borders after the Normandy landings on 6 June 1944 and the liberation of Paris on 25 August 1944. On the southern front, the Allies landed in southern France on 15 August 1944 and were preparing to attack the Gothic Line in Italy, where twenty German divisions were deployed. These divisions were commanded first by Field Marshal Albert Kesselring, and from 9 March 1945 by General Heinrich von Vietinghof.

In 1944, Stalin asked the Allies for a renewed Anglo-American war effort on the southern and western fronts to distract German divisions from the battlefields in the east, where the Soviet army was advancing.
However, this came at the cost of heavy losses in men and equipment. Strategically, Roosevelt considered the Italian front to be secondary to the war effort developing after the Normandy landings on 6 June 1944, which were advancing towards the Ruhr, the industrial heart of Germany.
Churchill, on the other hand, believed it was crucial to advance into the Balkans via the Italian peninsula, passing through Ljubljana and Trieste, before the Soviet Union could exert political influence over the region.
The Germans, for their part, considered it very dangerous for the Allies to occupy the Po Valley, home to Italy's major industries, and reach the Brenner Pass, which would take them to the southern border of the Third Reich.

Adolf Hitler ordered Kesselring to slow the Allied advance on the Italian peninsula as much as possible while awaiting the operational readiness of new weapons: the V1 and V2 missiles, and the ME262 — the world's first operational jet aircraft, developed by Third Reich scientists in Germany. Hitler was convinced that these weapons would turn the tide of the conflict.

In late August 1944, the US 5th Army and the British 8th Army arrived near the Gothic Line, which the Todt organisation had started constructing in 1943. Consisting of over 300 kilometres of minefields, barbed wire, trenches, shelters and bunkers housing men, artillery and machine guns, it was a formidable defence line.
The morphological characteristics of the mountains and the natural barriers of the northern Apennines were perfect for a defence line running across Italy from Massa Carrara to Pesaro. The Germans called this 30-kilometre-deep line of defence the Gothic Line (Gotenstellung). However, as the Allied armies approached, Kesselring renamed it the Green Line (Grüne Linie) in the summer of 1944.

The attack on the Gothic Line's first line of resistance, which began on 25 August 1944 by the 8th British Army on the Adriatic front, led to a breakthrough in many places. This was reached by the second line of resistance (Green Line 2) between September and October 1944. The Allied armies remained here throughout the winter of 1944–1945.

In January 1945, the 10th Mountain Division of the United States entered the front line in a theatre of battle which included the Monti della Riva, Monte Belvedere and Monte Castello, all of which were in German hands.
The offensive aimed to conquer the Monti della Riva first, followed by Monte Belvedere. It began on 18 February 1945 and saw the three regiments of the 10th Mountain Division advance along the mountain ridges of the Apennines.
By early March, they had reached the heights adjacent to Castel d'Aiano.

At the start of the Spring Offensive, the front line included the 4th US Army Corps, commanded by General Wills D. Crittenberger and deployed between the Tyrrhenian Sea and the Reno River.
The 10th Mountain Division was positioned between Monte Grande d'Aiano, Monte della Spe and Monte della Castellana. On 14 April 1945, the offensive began from this front line.
After overwhelming the German defences, the men of the 10th Mountain Division went on to control and block access to the Brenner Pass in the Alps above Lake Garda.

World War II on the Italian front ended unofficially on 28 April and officially on 2 May 1945, while Allied troops were advancing rapidly towards Trieste, Gorizia and Monfalcone, which were occupied by Tito's Yugoslav partisans.
Following a stalemate between the Allies and Tito, during which Winston Churchill authorised General Alexander to use all Allied divisions in Italy to repel the occupation of Italian cities,
Tito's forces withdrew on 12 June. However, the fate of Trieste remained unresolved until 1954.

Bibliography:
Ernest F. Fisher, Cassino to the Alps, 1989.
W. G. F. Jackson, The Battle for Italy. 1967.
Basil Henry Liddell Hart, History of a Defeat.
Field Marshal Viscount Alexander of Tunis, The Italian Campaign, 12 December 1944 to 2 May 1945. 1951.