'Giant I' was an Allied unrealised plan for an airborne crossing of the Volturno river on the western side of south central Italy by Major General Matthew B. Ridgway’s US 82^nd Airborne Division with the object of preventing German reinforcements from moving to the south through the Campanian plain to counterattack the Allied forces landing at Salerno in 'Avalanche' (September 1943).

When Italy agreed on 3 September 1943 to an armistice with the Allies, to become effective on 9 September, one of the agreement’s stipulations was that the Allies provide military support to Italy in defending Rome from German occupation. The resulting 'Giant II' was planned as the drop of one regiment of the 82^nd Airborne Division in the area to the north-west of Rome to assist four Italian divisions in seizing and holding the Italian capital.

This meant the cancellation of 'Giant I', which had been schemed as an airborne assault to seize crossings of the Volturno river during the Allied 'Avalanche' invasion of Italy at Salerno. However, as there remained strong Allied doubts about the Italian forces' willingness and ability to co-operate, and the distance of the mission far beyond the realistic possibility of support by the Allied military, Brigadier General Maxwell D. Taylor, the 82^nd Airborne Division’s artillery commander, was despatched to undertake a personal reconnaissance mission to Rome for the assessment of the prospects of success for 'Giant II'. Taylor’s report via radio on 8 September led to the postponement of the operation, which was then cancelled on the following day, even as the transport aircraft loaded with two battalions of the 504^th Parachute Infantry were being rewadied for take-off.

With 'Giant II' cancelled, 'Giant I' was reactivated for the drop of two battalions of the 504^th Parachute Infantry at Capua, on Highway 6 to the north-east of Salerno, during 13 September. The 'Orkan' (iv) German counterattack, starting on 12 September, then resulted in the contraction of the US southern sector of the 'Avalanche' beach-head, which was thus threatened with destruction.

As a result, 'Giant I' was cancelled and the 504^th Parachute Infantry was instead dropped into the beach-head on the night of September 13/14 using radar transponder beacons as a guide. On the following night the 505^th Parachute Infantry was also dropped into the beach-head as a further reinforcement. In all, 3,500 paratroopers made the most concentrated mass night drop in history, in the process providing the model for the US and British airborne landings in Normandy during 'Overlord' in June 1944. An additional drop on the night of 14/15 September was intended to deliver the 2/509^th Parachute Infantry to destroy a key bridge at Avellino, to the north of the beach-head in order to disrupt the movements of German motorised formations seeking to fall back from the now-secure beach-head, but was badly dispersed and failed to destroy the bridge before the Germans withdrew to the north.

It was also appreciated that the Allies' conventional crossing of the Volturno river in the following month might at the time offer great difficulties as the river would be in autumn spate and the Germans would likely have destroyed all the bridges across it; the plan was thus cancelled. In the event, on the third day after resuming the offensive on 12 October, Lieutenant General Mark W. Clark’s US 5^th Army managed to get its two major formations, Lieutenant General Sir Richard McCreery’s British X Corps and Major General John P. Lucas’s US VI Corps, across this formidable barrier and into the approaches of the yet more formidable 'Gustav-Linie' defences, namely the 'Barbara-Linie' positions.

After deciding that Generaloberst Heinrich-Gottfried von Vietinghoff-Scheel’s 10^th Army could not defeat the Allied 'Avalanche' landing at Salerno, and that the advance of General Sir Bernard Montgomery’s British 8^th Army, after the landing of Lieutenant General C. W. Allfrey’s British V Corps at Taranto ('Slapstick') and Brindisi, to the north along Italy’s eastern coast could now threaten the German rear, Generalfeldmarschall Albert Kesselring, the Oberbefehlshaber 'Süd', was compelled on 16 September to order the 10^th Army to disengage and pull back to the north. This decision resulted in renewed disagreement with Generalfeldmarschall Erwin Rommel, commanding the new but short-lived Heeresgruppe 'B' (19 July/26 November 1943) charged with the defence of northern Italy. Rommel wished to abandon Rome, but Kesselring believed that the Italian capital could be held with the aid of a defence line extending essentially between Formia on the west coast and Pescara on the eastern coast via Cassino, using the valleys of the Garigliano and Rapido rivers and the Abruzzi mountains, which reach a height of more than 9,000 ft (2745 m) at La Malella.

On 26 November Adolf Hitler recalled Rommel and appointed Kesselring head of a new Heeresgruppe 'C', and thus in overall operational command in Italy. Hitler also reduced the German strength in Italy by transferring Generalleutnant Maximilian Freiherr von Edelsheim’s 24^th Panzerdivision and SS-Obergruppenführer und General der Waffen-SS Joseph Dietrich’s 1^st SS Panzergrenadierdivision 'Leibstandarte SS Adolf Hitler' to the Eastern Front.

Kesselring allotted three divisions to the 10^th Army and the balance of the erstwhile Heeresgruppe 'B' in northern Italy went to form a new 14^th Army under General Eberhard von Mackensen. As the Germans pulled back, the Allies moved out of their lodgements in pursuit, the Americans in the west and the British in the east. By 8 October the two Allies had reached the lines of the Volturno and Biferno rivers respectively. Here they halted before launching their first attempts to break though the outer defences of the 'Gustav-Linie'.

Despite appalling weather, the 5^th Army moved off once again on 12 October, and the 8^th Army also got under way once more on 22 October. The first major obstacle which the 5^th Army faced was the Volturno river itself, which was in spate and whose bridges the Germans had demolished. However, by 15 October both of the 5^th Army’s formations, the British X Corps beside the sea and the US VI Corps farther inland, but then faced the 'Barbara-Linie' defences shielding the main works of the 'Bernhard-Linie' and 'Gustav-Linie'.