'Neptune' (iii) was the Allied designation for the amphibious landing, and to a lesser extent the associated naval operations, associated with 'Overlord' (6 June 1944).

The Allies assigned a number of codenames to the various operations involved in their 1944 invasion of north-western France. 'Overlord' was the name assigned to the establishment of a large-scale lodgement on the continent, while the first phase, which was the establishment of a secure foothold, was codenamed 'Neptune'. Thus 'Overlord' was the Allied land invasion of North-West Europe, which was started by the 'Neptune' (iii) initial assault phase: 'Neptune' (iii) began on 6 June (D-Day) and ended on 30 June 1944, by which date the Allies had established a firm foothold in Normandy; and 'Overlord' also began on 6 June but continued until the Allied forces crossed the Seine river on 19 August 1944.

The plans for the assault phase of 'Neptune' (iii) were extraordinarily complex, reflecting the ambitious nature of the assault plan, and were formulated a special staff under Admiral Sir Bertram Ramsay, the Naval Commander Expeditionary Force. The Allied plan was to land five divisions in the first wave (one division on each of the five selected beaches), which had been allocated in the ratio of three to Lieutenant General Sir Miles Dempsey’s British 2^nd Army on a 30-mile (48-km) front between the mouth of the Orne river and Port en Bessin, and two to Lieutenant General Omar N. Bradley’s US 1^st Army on a 20-mile (32-km) front between Port en Bessin and Les Dunes de Varreville.

To carry and escort the assault forces, the Allied naval command provided for two task forces, the Eastern Naval Task Force under Rear Admiral Sir Philip Vian for the British assault forces and the Western Naval Task Force under Rear Admiral Alan G. Kirk for the US assault forces. For their tasks the two commanders had very substantial naval assets.

The Western Naval Task Force, commanded by Kirk on board the US heavy cruiser Augusta, supported the US 1^st Army. As the first stage of the operation, 102 US, British and Allied minesweepers cleared the approaches in company with 16 buoy-layers to mark the approach lanes. Within the Western Naval Task Force, Force 'U', commanded by Rear Admiral Don P. Moon on board the headquarters ship Bayfield, supported Major General J. Lawton Collins’s VII Corps (Major General Raymond O. Barton’s 4^th Division in the U.1, U.2A, U.2B, U.3, U.3C, U.4, U.5A and U.5B convoys), and Force 'O', commanded by Rear Admiral John L. Hall on board the headquarters ship Ancon, supported Major General Leonard T. Gerow’s V Corps (Major General Clarence R. Huebner’s 1^st Division in the O.2A, O.2B, O.1, O.3, O.3C, O.4A, O.4B and O.5 convoys).

The warships and transports of both Force 'U' and Force 'O' approached their relevant assault areas during the night of 5/6 June. The convoys for 'Utah' and 'Omaha' Beaches comprised 16 attack transports, one dock landing ship, 106 tank landing ships, one rocket-firing landing ship, 15 control landing craft, 93 infantry landing craft, 350 tank landing craft, 34 support landing craft, 94 assault landing craft, 189 vehicle and personnel landing craft, 38 small support landing craft, 54 personnel landing craft and, for fire support, nine gun-armed landing craft, 11 flak landing craft, 14 rocket-armed tank landing craft, two medium support landing craft and 36 small support landing craft.

The support force for the 'Utah' Beach assault was Rear Admiral Morton L. Deyo’s Force 'A' with the US battleship Nevada, British monitor Erebus, US light cruisers Tuscaloosa and Quincy, British heavy cruiser Hawkins, British light anti-aircraft cruiser Black Prince, British light cruiser Enterprise, Dutch gunboat Soemba, US destroyers Hobson, Fitch, Forrest and Corry (Destroyer Division 20) and Butler, Shubrick, Herndon and Gherardi (Destroyer Division 34), and destroyer escorts Bares and Rich.

The support force for the 'Omaha' Beach assault was Rear Admiral C. F. Bryant’s Force 'C' with the US battleships Texas and Arkansas, British light cruiser Glasgow, Free French light cruisers Montcalm and Georges Leygues, US destroyers McCook, McCormick, Doyle, Baldwin, Harding, Satterlee and Thompson (Destroyer Squadron 18) and Emmons and the British escort destroyers Melbreak, Tanatside and Talybont.

As escorts for the approaching 'U' and 'O' convoys, the US destroyers Jeffers and Glennon (Destroyer Squadron 17) and Barton, O’Brien, Walke, Laffey and Meredith (Destroyer Squadron 60) and the Free French corvettes Aconit and Renoncule, and the US destroyers Frankford, Nelson, Murphy and Plunkett (Destroyer Squadron 33), British destroyers Vesper and Vidette, US destroyer escorts Borum, Amesbury and Blessman, and Free French frigates Aventure and Escarmouche were deployed for 'U' and 'O' respectively.

The British battleship Nelson, British light anti-aircraft cruiser Bellona, US destroyers Somers, Davis and Jouett (Destroyer Division 18) and Free French frigates Surprise and Decouverte were available as a reserve for the Western Naval Task Force.

On 6 June 23,250 troops were landed in the 'Utah' Beach area and 34,250 troops in the 'Omaha' Beach sectors.

The Eastern Naval Task Force, commanded by Vian in the British light anti-aircraft cruiser Scylla, supported Lieutenant General Sir Miles Dempsey’s British 2^nd Army. First, 102 British and Canadian minesweepers with 27 buoy-layers cleared and marked the approaches. During the night Force 'G', commanded by Commodore Douglas Pennant in the headquarters ship Bulolo, supported Lieutenant General G. C. Bucknall’s XXX Corps (Major General D. A. H. Graham’s 50^th Division in the G.1 to G.13 convoys, Force 'J' , commanded by Commodore G. N. Oliver in the headquarters ship Hilary, supported Major General R. F. L. Keller’s Canadian 3^rd Division in the J.1 to J.13 convoys, and Force 'S', commanded by Rear Admiral A. G. Talbot in the headquarters ship Largs, supported Lieutenant General J. T. Crocker’s I Corps with Major General T. G. Rennie’s British 3^rd Division in the S.1 to S.8 convoys approached their relevant assault areas during the night of 5/6 June.

The convoys for the 'Gold', 'Juno' and 'Sword' Beaches comprised 37 infantry landing ships, 130 tank landing ships, two rocket-armed landing ships, one dock landing ship, 66 control landing craft, 116 infantry landing craft, 39 support infantry landing craft, 487 tank landing craft, 66 support landing craft, 408 assault landing craft, 73 small support landing craft, 90 personnel landing craft and 10 support personnel landing craft. In addition, for fire support there were 16 gun-armed large landing craft, 22 rocket-armed tank landing craft, 14 large support landing craft, 24 medium support landing craft, 18 flak landing craft, 45 evacuation assault landing craft and 103 tank landing craft with armament.

The support force for the 'Gold' Beach assault force was Captain E. Longley-Cook’s Force 'K' comprising the British light cruisers Argonaut, Orion, Ajax and Emerald, Free Dutch gunboat Flores, British destroyers Grenville, Ulster, Ulysses, Undaunted, Undine, Urania, Urchin, Ursa and Jervis (25^th Destroyer Flotilla) and British escort destroyers Cattistock, Cottesmore, Pytchley and Free Polish Krakowiak.

The support force for the 'Juno' Beach assault force was Rear Admiral F. H. G. Dalrymple-Hamilton’s Force 'E' comprising the British light cruiser Belfast, British light anti-aircraft cruiser Diadem, and British destroyers Faulknor (8^th Destroyer Flotilla), Fury and Kempenfelt (27^th Destroyer Flotilla), Venus, Vigilant and Canadian Algonquin and Sioux, and escort destroyers Stevenstone, Bleasdale, Free French Combattante and Free Norwegian Glaisdale.

The support force for the 'Sword' Beach assault was Rear Admiral W. R. Patterson’s Force 'D' comprising the British battleships Warspite and Ramillies, monitor Roberts, light cruisers Mauritius, Arethusa, Frobisher, Danae and Free Polish Dragon, and destroyers Saumarez, Scorpion, Scourge, Serapis, Swift, Verulam, Virago, Kelvin and Free Norwegian Stord and Svenner, and escort destroyers Middleton, Eglinton and Free Polish Slazak.

Six destroyers, four sloops, eight frigates, 17 corvettes and 21 trawlers (British, Canadian, French and Norwegian) were deployed to escort the various G, J and S convoys. The British battleship Rodney and light anti-aircraft cruiser Sirius were among the ships which constituted the reserve for the Eastern Naval Task Force.

On 6 June 24,970 troops were landed on the 'Gold' Beach, 21,400 on the 'Juno' Beach and 28,845 on the 'Sword' Beach sectors.

To escort the first follow-up wave, Commodore C. D. Edgar’s Force 'B' was deployed in the west with the US destroyers Rodman, Ellyson and Hambleton, British destroyers Boadicea, Volunteer and Vimy, escort destroyers Brissenden and Wensleydale, and corvettes Azalea, Bluebell and Canadian Kitchener, and Rear Admiral W. E. Parry’s Force 'L' in the east with the escort destroyers Cotswold and Vivacious, frigates Chelmer and Halsted, corvettes Clematis, Godetia, Mignonette, Narcissus and Oxlip and three anti-submarine trawlers, 49 tank landing ships, 19 large infantry landing craft and 53 tank landing craft.

The British 10^th Destroyer Flotilla, comprising Eskimo, Javelin, Tartar, Ashanti, Canadian Haida and Huron, and Free Polish Błyskawica and Piorun, a group of frigates and eight groups of coastal forces with motor torpedo boats and motor gun boats covered the assault forces against attacks by surface craft in the western entrance to the English Channel. In the east, the same role was undertaken by the 17^th Destroyer Flotilla, consisting of Onslow, Onslaught, Offa, Oribi, Obedient, Orwell, Isis and Impulsive, and seven groups of coastal forces.

In addition, the escort forces also included the light cruisers Despatch, Ceres and Capetown, destroyers Kimberley, Opportune, Pathfinder, Beagle, Bulldog, Icarus, Campbell, Mackay, Montrose, Walpole, Windsor, Whitshed, Vanquisher, Versatile, Wanderer, Walker, Westcott, Wrestler, Caldwell, Leeds, Lincoln, Ramsey, Skate, Saladin and Sardonyx; escort destroyers Garth, Holderness, Meynell, Avon Vale, Belvoir, Goathland and Free French Combattante, sloops Scarborough, Rochester, Hart, Kite, Lapwing, Lark, Magpie and Pheasant, frigates Deveron and Nene and ex-US destroyer escorts Cubitt, Dakins, Ekins, Holmes, Lawford, Retalick, Stayner and Thornborough, and corvettes Puffin, Armeria, Balsam, Burdock, Buttercup, Campanula, Celandine, Dianthus, Gentian, Heather, Honeysuckle, Lavender, Nasturtium, Pennywort, Primrose, Starwort, Sunflower and Wallflower, Free French Commandant Estienne d’Orves, Free Norwegian Acanthus, Eglantine, Potentilla and Rose, and Canadian Alberni, Baddeck, Battleford, Calgary, Camrose, Drumheller, Lindsay, Louisburg, Lunenburg, Mimico, Moosejaw, Port Arthur, Prescott, Regina, Rimouski, Summerside, Trentonian and Woodstock.

Altogether, the Allies used seven battleships, two monitors, 23 cruisers, three gunboats, 105 destroyers and 1,073 smaller naval vessels.

To oppose these forces, on 6 June the Germans had in the English Channel area just five torpedo boats, 34 operational and five non-operational Schnellboote, 163 minesweepers and motor minesweepers, 57 patrol boats and 42 gun carriers, while on the Atlantic coast between Brest in the north and Bayonne in the south they had five destroyers, one torpedo boat, 146 minesweepers and motor minesweepers, and 59 patrol boats.

An important part of 'Neptune' (iii) was the isolation of the invasion routes and beaches from any intervention by elements of the German navy. The responsibility for this all-important task was assigned to Admiral Sir Bruce Fraser’s Home Fleet. Two principal German naval threats were foreseen. The first was surface attack by German major warships from anchorages in Norway and the Baltic. This did not materialise since, by this period in the middle of 1944, the battle-cruiser and pocket battleships were damaged and the cruisers were used primarily for training and, moreover, the German navy was suffering the effects of acute fuel shortages. The inactivity may also have resulted from Adolf Hitler’s total disillusionment with the Kriegsmarine.

Nonetheless, the Royal Navy had strong forces available to repel any German attempt to intervene with surface warships.

The second perceived threat was that of U-boats transferred from the Atlantic. Air surveillance from three escort carriers and Air Chief Marshal Sir William Sholto Douglas’s RAF Coastal Command allowed the establishment and maintenance of an air cordon well to the west of Land’s End. Few U-boats were spotted and most of the escort groups were moved nearer to the landings. Further efforts were made to seal the English Channel against German naval forces from the German-held ports of Brittany and the Bay of Biscay. Minefields were laid to force German ships out of the range of land-based air cover so that they could be attacked by Allied destroyer flotillas. Again, German naval activity was minor, but on 4 July four German destroyers were either sunk or forced back to Brest.

The Strait of Dover was closed by minefields, naval and air patrols, radar surveillance, and effective bombing raids on German ports. Local German naval forces were small, but could have been reinforced from the Baltic Sea. The German naval effort at the eastern end of the English Channel was concentrated on protection of the Pas de Calais against the landings the Germans were expecting as a result of the 'Fortitude' deception scheme, and no attempt was made to force the British blockade. The screening operation destroyed only a few German ships, but the major objective was nonetheless achieved. There were no U-boat attacks against Allied shipping and few attempts by surface ships.