'Impact' was a British two-phase operation ('Impact Plain' and 'Impact Royal') to widen and deepen the British bridgehead in the 'Wedge' on the south-western side of Lake Comacchio and also to cross Lake Comacchio and take Longastrino and Menate in northern Italy as a prelude to the Battle of the Argenta Gap (10/11 April 1945).

By the winter of 1944/45 the Allied armies had penetrated the main defences of the 'Gotisch-Linie' but had not been able to exploit this fact by breaking out into the wide valley of the Po river’s lower reaches. A side-by-side pair of major offensives were planned for a time early in 1945 as the US 'Craftsman' to take Bologna and advance north-west to Milan and Turin, and the British 'Buckland' to cross the Senio river and advance north past the western side of Lake Comacchio and then north-east to take Venice and Trieste. These paired undertaking were schemed for the spring of 1945, when the return of drier conditions would make it possible for the Allies to take full advantage of the superiority of their air armoured and motorised forces.

Devised under the control of Field Marshal the Hon. Sir Harold Alexander, the Supreme Allied Commander Mediterranean Theatre, by General Mark W. Clark’s Allied 15^th Army Group as part of the British forces' final 'Buckland' offensive in Italy, 'Impact Plain' was evolved as a means for Lieutenant General Sir Richard McCreery’s 8^th Army to get past the formidable German defences in the region and through the bottleneck of the Argenta gap, in which the natural water obstacles constituted by the Reno river and Lake Comacchio were compounded by a massive minefield in the area of Bastia (to the west of Lake Comacchio en route to the Argenta gap) and by the powerful capabilities of General Richard Heidrich’s I Fallschirmkorps and General Gerhard Graf von Schwerin-Krosigk’s LXXVI Panzerkorps 1 of General Traugott Herr’s 10^th Army within Generaloberst Heinrich-Gottfried von Vietinghoff-Scheel’s Heeresgruppe 'C'.

It was therefore decided that the forces of Lieutenant General C. F. Keightley’s V Corps should break through the Argenta gap, aided by an amphibious 'right hook' across Lake Comacchio.

In its complete complexity, 'Buckland' called for a frontal attack by the New Zealand 2^nd Division, 78^th Division and Indian 8^th Division through the 98^th Volksgrenadierdivision toward Massa Lombarda on 1 April to draw the defence’s attention to this sector just as Brigadier R. J. F. Tod’s 2^nd Commando Brigade advanced along the 'Spit' to the east of Lake Comacchio and through von Heygendorff’s 162^nd Division toward Porto Garibaldi with the intention of drawing German attention to the east again, or alternatively of exploiting any local advantage. Then on 4 April, with German attention split between Massa Lombardo and 'the Spit', a party of the Special Boat Squadron would seize the islands in Lake Comacchio to protect the open flank of the 'right hook' proper on 6 April by the 56^th Division (one brigade) in 'Lever', supported by Brigadier C. H. V. Pritchard’s 2^nd Parachute Brigade and Tod’s 2^nd Commando Brigade (Nos 2 and 9 Commandos and Nos 40 and 43 Royal Marine Commandos), whose task was to cross Lake Comacchio in amphibious craft in 'Impact Plain' to emerge behind the left flank of Jost’s 42^nd Jägerdivision, to take the vital bridge over the Reno at Bastia and to hold the Argenta gap until the arrival of the Indian 8^th Division and British 78^th Division.

To disguise this major undertaking the British designed the 'Roast' deception attack across 'the Spit' of land dividing the eastern shore of Lake Comacchio and the west coast of the Adriatic Sea. This would secure the 8^th Army’s eastern flank, allow the operational evaluation of the newly arrived LVT amphibious tracked landing craft for subsequent larger operations in the muddy and difficult conditions of Lake Comacchio. The operation was successful, although the LVTs failed dismally, becoming hopelessly bogged down in the shallows of the lake, which were neither water nor earth. The 8^th Army also hoped that the Germans would see 'Roast' as an attempt to break through to the mouth of the Po river and so switch their forces eastward away from the area in which the main thrust was to be delivered on the other side of Lake Comacchio by Major General J. Y. Whitfield’s 56^th Division, Major General D. Russell’s Indian 8^th Division, Major General R. K. Arbuthnott’s 78^th Division and Lieutenant General Sir Bernard Freyberg’s New Zealand 2^nd Division.

Two nights later further actions by the Special Boat Service aided by Italian partisans of AlbertoBardi’s 28^th Brigata Garibaldi 'Mario Gordini' captured islands in the middle of the lake.

The whole scheme worked like clockwork, and so successful was the Allied deception scheme for the entire offensive that von Vietinghoff-Scheel came to believe that the main Allied effort was to be an amphibious operation against the mouth of the Po river, and so sent half of his reserve force, Generalmajor Fritz Polack’s 29^th Panzergrenadierdivision, from its position near Bologna, where it posed a distinct threat to the plans of Lieutenant General Lucian K. Truscott’s US 5^th Army, to the coast near Venice.

In overall terms, therefore, 'Impact Plain' was a key element of the Battle of the Argenta Gap, which was fought between 12 and 19 April by Lieutenant General C. F. Keightley’s British V Corps (Whitfield’s 56^th Division and Arbuthnott’s 78^th Division plus supporting units including an armoured brigade for each division) and General Gerhard Schwerin’s LXXVI Panzerkorps (Jost’s 42^nd Jägerdivision, von Heygendorff’s162^nd Division and Weber’s 362^nd Division with Polack’s 29^th Panzergrenadierdivision available from army reserve).

The 56^th Division, on the right of the V Corps, also made a preliminary attack on 5/6 April to secure its starting line for the Argenta gap operation. By 8 April, after meeting stiff resistance, it had completed its task in clearing the Wedge, an area on the western side of Lake Comacchio between its southern tip, where the Reno river flows into it, and the Fossa di Navigazione. The 8^th Army’s main offensive across the Senio river started on 9 April, and by 12 April the assault units had advanced to consolidate across the Santerno river and allow the 78^th Division to pass through toward the Reno river and the Argenta gap. This last was a well defended strip of land some 2 miles (3.2 km) wide and 8 miles (13 km) deep between Lake Comacchio and the Lombardy marshes south of Ferrara near the south bank of the Po river.

It was during the night of 10/11 April that V Corps launched 'Impact Plain' to widen and deepen its bridgehead in 'the Wedge': No. 40 (Royal Marine) Commando advanced along the raised causeway bordering the lake while Brigadier W. H. Stratton’s 169^th Brigade of the 56^th Division moved forward on the commandos' left across the flooded margins of the lake with two battalions in LVTs. Brigadier J. Scott-Elliott’s 167^th Brigade of the same division advanced from 'the Wedge' along the flood banks of the Reno river. The commandos met strong resistance at the bridge to the north-east of Menate and suffered many casualties, but were ably supported from the air as they took their objective.

The 42^nd Jägerdivision was apparently surprised by the British amphibious capacity and was unnerved by the emergence of the LVTs from the water, so by dawn on 12 April all three columns had progressed some 4 miles (6.4 km), linking in the area of Menate and Longastrino area. The 169^th Brigade then pushed along the road toward Filo, and the 167^th Brigade continued to the west along the Reno, rolling up the German defences as far as the confluence with the Santerno river, where it linked with Generale di Brigata Clemente Primiera’s Gruppo di Combattimentio 'Cremona', which had advanced from the south.

At this time von Vietinghoff-Scheel ordered the 29^th Panzergrenadierdivision to move to the south and reinforce the defence of the Argenta gap. The division’s 15^th Panzergrenadierregiment reinforced the 42^nd Jägerdivision on 12 April, but the rest of the division had been positioned to the north of the Po and, delayed by air attacks as well as fuel shortages, was not in position until 14 April.

Early on 13 April Brigadier T. P. D. Scott’s 38^th Brigade of the 78^th Division attacked to the north from the bridgehead of the Indian 8^th Division across the Santerno river with the task of establishing a bridgehead across the Reno river at Bastia, in the mouth of the Argenta gap.

Meanwhile, to the right, the 56^th Division launched the 'Impact Royal' second phase of its operation. This took No. 9 Commando and Brigadier M. D. Erskine’s 24^th Guards Brigade up the flooded margins of Lake Comacchio in LVTs to concentrate near Chiesa del Bando, 6 miles (9.7 km) to the north-west of Menate and thus threaten Argenta, which lay some 3 miles (4.8 km) to the south-west of this objective. The British established a foothold on the Fossa Marina, a canal running roughly east/west from Argenta to the lake and 1 mile (1.6 km) short of their objective, but the newly arrived 15^th Panzergrenadierregiment then checked further progress.

An attempt to take the bridge across the Fossa on 14 April was defeated. During the morning of 14 April forward elements of Scott’s 38^th Brigade crossed the bridge over the Reno river at Bastia but was then driven back by an armoured counterattack. The British then decided to limit themselves in the short term to completing the clearance of the area to the south of the Reno river and await the arrival of Scott-Elliott’s 167^th Brigade which was advancing along both banks of the river and would soon pose a threat to the flank of the Germans to the north of the river in Bastia, and therefore compel them to pull back.

Rather than wait for the bridge at Bastia to be cleared, Keightley ordered Brigadier G. E. Thubron’s 11^th Brigade of the 78^th Division to use the 56^th Division’s bridges over the Reno river and advance as rapidly as possible toward Argenta. On 15/16 April the 56^th Division renewed its attack on the Fossa Marina, but once more failed.

The cumulative effect of heavy air attacks since 13 April took their toll on the Germans, however, and on the night of 16 April the 24^th Guards Brigade crossed the canal with relatively little trouble although its advance was once again checked only a short distance to the north of the canal.

On the western side of the Argenta gap the 11^th Brigade was able to get across the Fossa Marina east of Argenta. The 2/Lancashire Fusiliers was able to hold on to a small bridgehead despite the Germans' best endeavours, and engineers positioned ARK armoured mobile bridges to allow supporting tanks to cross the canal. On 17 April Scott’s 38^th Brigade arrived from Bastia and passed through the 11^th Brigade’s bridgehead to widen this so that there would be space to facilitate an armoured break-out. By dark the brigade had advanced 1,000 yards (915 m) against determined resistance, working around the rear of Argenta. Meanwhile the 11^th Brigade advanced into the town to clear it with the support of Crocodile flamethrowing tanks. An armoured counterattack early on 18 April was forced back toward the 38^th Brigade.

Meanwhile on the western outskirts of the town the bridges over the Reno river had been captured by Tod’s 2^nd Commando Brigade, which had been advancing up the river from Bastia. German counterattacks on the bridges were broken up by the fire of the supporting artillery. Also on 17 April, Stratton’s 169^th Brigade of the 56^th Division passed through the 11^th Brigade’s bridgehead across the Fossa Marina to drive toward the east, clear the canal’s northern bank and link with the 24^th Guards Brigade.

On 18 April the 78^th Division brought forward Brigadier G. R. D. Musson’s 36^th Brigade from reserve to pass through the 38^th Brigade and undertake a series of right hooks to the north of Argenta. By dawn the brigade had reached Consandolo, about 4 miles (6.4 km) to the north-west of Argenta, where the Germans maintained their hold until a time well into the afternoon. Meanwhile a mobile force under the command of Brigadier J. F. B. Combe’s 2^nd Armoured Brigade, attached to the 78^th Division, and comprising one infantry battalion, one tank regiment, one regiment of armoured personnel carriers and supporting self-propelled guns and assault engineers, was brought forward and, bypassing Consandolo, secured a bridgehead over the Fossa Benvignante about 1 mile (1.6 km) to the north of the town. Advanced elements then pushed forward and before dark overran the artillery lines of the 42^nd Jägerdivision.

The advance of the 78^th Division created pressure on the flank of the German units facing the 56^th Division, and by 12.00 on 18 April the 69^th Brigade had detected a lessening of the resistance in front of it. The brigade pushed on and advanced to the Fossa Benvignante, where it captured a bridge which the Germans had not blown. To the 69^th Brigade’s right the 24^th Guards Brigade at last cleared Chiesa del Bando and also advanced toward the Fossa Benvignante.

With the 56^th Division and 78^th Division now clear of the Argenta gap’s northern end, Major General H. Murray’s British 6^th Armoured Division was released from 8^th Army reserve and advanced through the left wing of the 78^th Division before wheeling left to surge toward the north-west along the line of the Reno river to Bondeno, where it met elements of the US 5^th Army advancing north from this army’s advance to the west of Bologna, thereby completely the encirclement of the German forces holding Bologna.

Effective Allied bombing of the crossings of the Po and shortage of fuel left much of Heeresgruppe 'C' (many men and almost all of its heavy equipment and weapons) stranded on the southern side of the river. As the Argenta gap opened to British exploitation, the whole German defence of north-eastern Italy collapsed, and General dywizji Władisław Anders’s Polish II Corps, Lieutenant General Sir John Hawkesworth’s British X Corps and Lieutenant General Sir John Harding’s British XIII Corps were able to press ahead on the 8^th Army’s left flank between Bologna and Ferrara. By 23 April the Allied line had advanced to positions just south of the Po river as far to the west as Modena.

On 29 April an instrument of surrender was signed by German emissaries at the Allied headquarters in Caserta, and hostilities formally ceased on 2 May.