Allied superiority at sea and in the air over it made the Normandy invasion possible. The British and American navies and air forces were so successful against Grand-Adm. Doenitzs U-boats in 1943 that his fleet was unable to prevent the Allied buildup in Britain and early in 1944 was clearly on the defensive in the Atlantic Ocean. The battleships and cruisers of the German Navy, which once dared to operate from French ports and even to steam brazenly through the narrow English Channel, had either been sunk or penned into their ports.
Even for the less-easily detected German combatants such as destroyers. E-boats, and minelayers, it became foolhardy, except on moonless nights, to venture into the Channel.
The same applied to the Luftwaffe, whose fighters were already fully committed defending vital industrial plants, transportation links, and military resources ashore from the depredations of the Allied bomber fleets. In short, by June 1944, the Allies owned the Channel between England and France and the air over it.
To keep the enemy on the defensive, beginning on April 17, British minelayers, motor launches motor torpedo boats, and aircraft laid almost
7,000 mines off the enemy’s principal naval bases at Boulogne, Le Havre, Cherbourg and Brest and along much of the Breton coastline. This operation was not without cost. German torpedo boats sank an escorting ship, the Canadian destroyer Athabascan, and a British motor torpedo boat (MTB) ran afoul of an enemy mine.
In spite of these Allied measures, the Germans still had lethal forces that they could throw into battle to defeat an Allied landing The Allies expected their foe to spare no man or machine in their defense of the Atlantic Wall. The most immediate threat to the invasion armada would come from 50 U-boats based in Brittany and approximately 100 fast attack craft (E-boats and R-boats) and midget submarines concentrated in the French Atlantic ports. The Allies would also have to contend with German sea mines.
Protection of the invasion fleet’s flanks in the waters between Cornwall and Brittany on the west and the Dover-Calais narrows on the east was provided by 24 destroyers and frigates and approximately 22 flotillas of motor torpedo boats, motor and steam gun boats, and motor launches.
The Allies took measures to combat the enemy’s naval threats. The RAF Coastal Command reinforced its air groups patrolling the waters between Ireland and the Brittany Peninsula and off Scotland. There were so many aircraft in 19 Group, responsible for the western approaches to Normandy, that each segment of the broad patrol area was overflown every half-hour, day and night. Most of the U-boats, if they escaped attack, would be forced to proceed submerged and this would exhaust their batteries long before they reached the cross-channel invasion routes.
The Coastal Command units off Scodand drew first blood in the campaign against the U-boats. On May 16, Doenitz ordered many of the boats operating in Norway to deploy to the Bay of Biscay off France. For the next 18 days the submarines ran the gaundet set up by the British in the North Atlantic. Coastal Command aircraft sank 7 of the 22 U-boats they spotted.
Other Allied air forces prepared to cover the passage of the naval armada across the Channel and to protect the vulnerable transport and glider aircraft ferrying the three US and British airborne divisions to Normandy, For this task, the Allies deployed almost 2,000 fighter aircraft organized in 171 squadrons. These units would be controlled from fighter direction stations located in England, on board three LSTs in the Channel, and eventually ashore in Normandy. Swarms of fighters would also fly covering missions high above and all around the invasion site and maintain this presence, day and night, throughout Neptune.
To avoid Allied sailors mistaking paratroop-laden transports for German fighters and bombers and shooting them down, which occurred too frequently during the invasion of Sicily, the command established air corridors that would skirt the invasion armada to the east. In addition, sailors were schooled in aircraft recognition.
For the invasion armada to converge on assembly points off southern England, form into groups, and proceed through established lanes to the French coast without ships going off course or straying into German or Allied minefields, it was essential that there be navigation aids all along the route. These aids included motor launches and floating beacons situated at critical points.
The destruction of enemy coastal guns and beach defenses would begin with strikes by Allied bombers in the early hours of D-Day and reach a crescendo in the hour before the assault. Furthermore, each of the five assault forces was assigned a group of bombarding ships and these groups would concentrate their fire on the 24 enemy batteries emplaced around the landing beaches.
Each of the larger bombarding ships was assigned an enemy coastal gun site, normally containing four guns, as her first priority. The German batteries usually armed with the long-range, anti-ship 4.9-inch and 6.1-inch guns were protected by seven feet (2m) of reinforced concrete. Wheeled artillery pieces, smaller guns which could shift their fire from naval vessels to troops on the beach, were positioned both in concrete casemates and in the open.
In late May thousands of warships, amphibious landing ships, transports, and auxiliary ships and craft, and hundreds of thousands of Allied troops converged on the ports, rivers, and open roadsteads of southern England. Armored vehicles and trucks boarded ships directly from the beach by way of newly built hard stands, appropriately called “hards.” The British and American infantry assault divisions, and the naval forces that would enable the Allied host to cross the treacherous Channel, assembled in England in the same order in which they would storm the Normandy coast. The US 4th Infantry Division, slated to assault Utah Beach on the Cotentin Peninsula, gathered on the Devon coast at Salcombe, Dartmouth, and Brixham. To the east in Dorset at Portland, Weymouth and Poole was the US 1st Infantry Division, which would seize Omaha Beach. Continuing on to the east in Hampshire was the British 50th (Northumbrian) Infantry Division at Southampton, the Canadian 3rd Infantry Division around Southampton and Portsmouth, and the British 3rd Infantry Division near Portsmouth and at Shoreham and Newhaven. The last three divisions would storm ashore on Gold, Juno, and Beaches.
The reinforcing or follow-on divisions and their naval escorts were located on the wings of the Allied invasion assembly, with the US 29th Infantry Division to the west on the Devon and Cornish coasts at Plymouth and Falmouth and the British 7th Armoured and 51st (Highland) Divisions around the Thames Estuary at Tilbury and Felixstowe. The battleships and other heavy units of the bombardment groups swayed at anchor in Belfast, Northern Ireland and in Scotland’s Clyde Estuary.
One of the keys to a successful amphibious operation is to select the day and time of day when the weather and the tides are optimum. To increase the chances of surprise and to provide the invading forces with concealment from enemy guns, Allied army leaders advocated a night landing. Naval officers, however, made it clear that a landing during daylight hours was absolutely essential to ensure accurate naval gunfire support and to coordinate launching and reloading of landing craft. Moreover, Allied underwater demolition teams and beach engineers needed light to clear lanes to the beach through the German obstacles.
Allied planners wanted the assault to begin three or four hours before high tide and be preceded by a moonlit night that would ease the navigation problems of the thousands of vessels and aircraft making the Channel, and help the airborne troops maintain some order as they dropped behind the beach defenses. There were only three or four days each month in the spring when all these conditions prevailed in the Channel between England and Normandy. Taking all these factors into consideration, on May 23, General Eisenhower established D-Day as June 5. Two days later, this critical information was passed to naval commanders when Adm. Ramsay ordered them to open their sealed operation orders. Simultaneously, he prohibited the crews of the invasion flotilla from sending out mail, using the telephone, or wiring telegrams. Then, on May 28, all crews and troops were “sealed” in their ships.
A few security breaches occurred anyway. Several communication transmissions connected D-Day with June 5, and tugs were inadvertently issued charts of the Bay of the Seine. Despite these lapses the Germans remained unaware of the invasion date.
On June 1, Adm. Ramsay, from his “battle headquarters” at Southwick House near Portsmouth, took command of Allied naval forces. The following evening two British midget submarines, X-20 and X-23 (markers for Forces J and S), proceeded out to sea from Portsmouth, and the bombardment ships sortied from the Clyde. In the words of Stephen W. Roskill, the British naval historian, “like the giant flywheel of a power plant its first movement was barely perceptible, but with every revolution it gained further momentum until it was running smoothly at the speed for which it had been designed.”
On Sunday, June 4, however, convinced by his meteorologists that the weather would be too rough the following day, Gen. Eisenhower postponed the operation for 24 hours. Convoys that had already set sail had to return to their anchorages; all except one large US convoy that received the recall message when approaching French waters.
Time was running out for Gen. Eisenhower, If the invasion could not be launched on June 6, tide and moonlight conditions would not be suitable for another two weeks. The troops, many seasick, could not be confined too long on board crowded ships. To let them disembark would compromise security. Finally, at 04:15 on June 5, the Supreme Commander Allied Expeditionary Force started the monumental enterprise with the simple words, “OK, we’ll go.”
That same morning Adm. Ramsay boarded a motor-torpedo boat which proceeded to a vantage point off the coast of southern England. The most powerful invasion fleet the world had ever seen began steaming past him en route to the hostile shore of occupied France. Allied minesweepers cleared coastal waters along the coast of southern England and four channels leading to a general assembly area, nicknamed “Piccadilly Circus”, eight miles (12km) southeast of the Isle of Wight. From this staging point, five miles (8km) in radius, the Allied naval units established five marked channels, altogether known as “the Spout,” south toward France. The Germans had laid a mine barrier to the west of Dieppe and south of the 50th parallel, through which the Allied minesweepers cleared and marked ten channels with lighted buoys, each channel being 400 to l,200yards (360—1,080m) wide. The Western Task Force was allotted four lanes and the Eastern Task Force six.
Once the invasion convoys crossed into the assault area off Normandy, the minesweepers swept ahead of the task forces to clear areas from which warships would bombard coastal targets and amphibious ships would lower away landing craft for the run-in to the beach. During the operations, German mines damaged two British ships, the destroyer Wrestler and an LST. By skilfully clearing passages through the German minefield, however. Allied minesweepers made it possible for the thousands of other vessels in the armada to reach the far shore safely.
Equally important to the success of Neptune were the hundreds of buoy-laying vessels, salvage and rescue vessels, colliers, water tankers, telephone cable-laying vessels, despatch boats, smoke-making trawlers, and landing craft repair vessels.
Aside from mines, the elements, including a sea with large, foam-crested waves and a 16- to 20-knot wind from the west, posed the greatest hazard to the Allied naval force. A small number of light craft, most of which were being towed across the Channel, sank before reaching the far shore.
The heavy seas that caused thousands of seasick soldiers to long for landfall also induced the German command to suspend the daily surface ship patrols of the Channel. In addition, Allied bombers destroyed the enemy’s radar stations at Cape Barfleur and elsev/here on the invasion coast. The Allies also jammed enemy radars. No one in the armada compromised radio security. Finally, Allied forces staged feints and other ruses to divert enemy attention from Normandy. As a result, the Allies achieved complete strategic and tactical surprise. The Germans learned of the Allied presence only when their few remaining coastal radars picked up surface contacts around 03:00 on the morning of June 6.
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