Operation Neptune was a combined British and United States undertaking by all Services of both nations, assisted by their Allies, whose object was defined as:
“to carry out an operation from the United Kingdom to secure a lodgement on the Continent from which further offensive operations can be developed. This lodgement area must contain sufficient port facilities to maintain a force of 26 to 30 divisions and to enable this force to be augmented by follow-up formations at the rate of from three to five divisions a month”.
Neptune was thus the assault phase of Operation Overlord, which was the general plan for the liberation of north-west Europe; a common misconception has resulted in the latter code-word being commonly (but incorrectly) used for the invasion.
Detailed planning for a major cross-Channel amphibious operation began in late April 1942 under the codename” Round-Up”. Admiral Sir Bertram Ramsay was appointed the naval commander and, although he soon left to plan and command the 1942-43 series of Mediterranean amphibious operations, a small staff continued to plan the necessary UK infrastructure, providing headquarters at Portsmouth and Plymouth, landing craft bases and maintenance facilities and loading yards.
The Seine Bay was provisionally chosen as the assault area as early as January 1943. In May 1943, the Commander-in-Chief, Portsmouth (Admiral Sir Charles Little) was appointed Naval CinC (designate) for the invasion of Europe and made responsible for the preparation of the naval plan. At the end of June, a conference (“Rattle”) was held in London, chaired by Vice Admiral Lord Louis Mountbatten and attended by, among others, US and Canadian Army representatives. This conference reached definite conclusions as to future planning, training and the provision of equipment, including the need for artificial harbours. Six weeks later, in August 1943, the plan which followed the conclusions of Rattle was approved by the Combined Chiefs of Staff at the Quebec Conference.
Admiral Ramsay returned to resume naval command as Allied Naval Commander Expeditionary Force (ANCXF) of the operation in October 1943 and although detailed joint planning began on 15 December 1943, much had to be changed after the subsequent appointment of General Montgomery as CinC 21st Army Group. Planning was recommenced on 14 January 1944 and the initial joint plan was issued on 1 February, followed on 15 February by the naval outline. Provisional naval orders were issued on 2 April and, on a very limited scale, sealed orders on 24 April. Two days later, ANCXF moved to Southwick Park and on 12 May the naval plan was “frozen”.
Earlier, on 8 May, Admiral Ramsay informed General Eisenhower that 5 or 6 June were the earliest acceptable dates, with 7 June in case of extreme necessity. Holders were ordered to open the operation orders on 25 May and on the 28th were informed that the Supreme Commander had decided on 5 June as ‘D’-Day. At 0415 on 4 June the decision was taken to delay by 24 hours, in the hope of better weather.
Five sectors were targeted, three in the Eastern (British) Task Force Area and two in the Western (US) Task Force Area:
“Sword” between Ouistreham, at the mouth of the Orne and Lion Sur Mer
“Juno” between St Aubin and Ver-sur-Mer (around Courseulles)
“Gold” between Ver-sur-Mer and Port-en-Bessin (around Arromanches)
“Omaha” between Port-en-Bessin and Isigny (around St Laurent)
“Utah” between Quinelle and Isigny (near St Martin de Varreville)
The beaches were not assaulted along their full lengths ‘gap-filling’ being left to follow-up formations. The left flank, east of the Orne, was secured by an airborne drop on ‘Pegasus Bridge’.