The Allied Invasion of Italy was the beginning of campaign in mainland Italy, taking place shortly after the occupation of Sicily by the Allies.
On September 3, 1943, the new Italian leadership signed an armistice with the Allies, but Hitler was not conceiving to lose Italy. The Germans began to disarm Italian units and occupied the country. The Wehrmacht had in all southern Italy, under Albert Kesselring, three Panzer divisions, two Panzergrenadier divisions and one Fallschirmjäger division.
The day the Italians signed the armistice, the British landed in Calabria, located at the southern tip of mainland Italy, and a few days later in the port of Taranto, in southeastern Italy. Having insufficient forces, the Germans had left just a small rearguard to delay the Allied advance, especially by destroying infrastructure.
However, other Allied troops, which landed near the port of Salerno in southwestern Italy, had to encounter a more serious opposition. The landings at Salerno, code-named Operation Avalanche, began on September 9, having as ultimate goal the city of Naples.
The U.S. 5th Army, under General Mark W. Clark, landed in the first wave three infantry divisions, two British and one American. Groups of U.S. Rangers and British Commandos occupied the northern mountain passes leading to Naples. In the early days, the Allies managed to consolidate their position, advancing a few miles inland, despite opposition from the 16th Panzer Division.
Kesselring answered quickly bringing reinforcements, before the Allies have time to land enough forces. On September 13, the Germans counterattacked, inflicting significant losses to the U.S. 36th Infantry Division.
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The U.S. 82nd Airborne Division was brought urgently to strengthen the defense. Only the powerful Allied naval and air support made the Germans to escape a decisive victory. Fortunately for the Allies, the British Eighth Army of Montgomery was approaching, coming from Calabria, which put extra pressure on the Germans. Beyond the military situation, the Germans regarded Southern Italy as strategically unimportant, so they retreated in order towards Rome on successive lines of defense.
The Allies had been able to open a front in Southern Europe, eliminating Italy as Axis power and attracting precious German military resources, but the Italian campaign would prove long and costly in terms of human lives.