Nazi Germany had conquered nearly all of continental Europe by June 1940. It had invaded the Soviet Union on June 22, 1941, but seriously miscalculated Soviet resistance, the poor condition of roads, and its own ability to supply its forward troops. Unprepared for the harsh Russian winter because they felt the Soviet Union would have already collapsed, German forces suffered nearly a 25 percent casualty rate due to weather alone. By January 1942, the Nazi offensive had bogged down. While it was still not clear that the Soviet Union would be able to mount any offensives in the spring and summer of 1942, it was clear that Nazi Germany had lost and would eventually be pushed back within its own borders. What was unknown was when that would occur.
President Franklin D. Roosevelt and Prime Minister Winston Churchill met in the Moroccan city of Casablanca from January 14 to 24, 1943. Josef Stalin did not come, as the Battle of Stalingrad was still in progress. At this meeting, Churchill and Roosevelt agreed to invade continental Europe "some time soon." Stalin wanted a promise of an invasion by the end of 1943, but this promise was not forthcoming. The three world leaders did meet in Tehran from November 28 and December 1, 1943. Again, a pledge to invade continental Europe was made.
The British had established a unit known as the Chief of Staff of Supreme Allied Command (COSSAC). At Teheran, COSSAC presented various plans for the Western Allies (the U.S., U.K., Free French, Free Poles, Canadians, etc.) to launch an invasion of Europe. Planning began immediately. In December 1943, the Western Allies formed SHAEF -- the Supreme Headquarters Allied Expeditionary Force, led by the U.S. General, Dwight D. Eisenhower. General Sir Bernard Law Montgomery was named commander of all of the ground invasion forces and given charge of developing the invasion plan.
The Dieppe Raid had taught the Allies a host of lessons about amphibious invasion. They learned that assaulting a seaport was a bad idea; it was far better to land at a lonely beach. They learned that simple (rather than complex) plans were the key to success. They learned that the utmost secrecy had to be maintained. They learned that they needed a host of new, creative methods for breaching sea walls, anti-tank barriers, and getting tanks ashore. They had to provide supporting fire from both sea and shore, and be able to overcome strong German resistance.
But where was the best spot to invade?
Providing air cover was critical. A raid could occur with only moderate air support. But landing vast numbers of troops, tanks, vehicles, and war materiél would require near-absolute air superiority. Unfortunately, British fighters had short operating ranges. This greatly limited the number of potential landing sites. Geography (high cliffs, huge tidal flats, not enough deep water to allow ships to come close, etc.) reduced the number of choices even further.
Only two sites remained: Calais and Normandy. Calais seemed the most suitable at first. It was the closest to Great Britain, and could be covered from the air. It had some natural harbors, and could easily be approached by sea. A breakout by Allied troops would enable the invading armies to sweep across southern Belgium and toward Germany. Along the way, the Allies could liberate major cities like Lille, Paris, Brussels, and Antwerp and get a major morale boost.
Normandy seemed less likely. Cherbourg, a major port, was located on the Normandy coast and heavily defended. High cliffs confronted the beaches. There were no natural harbors. And if the Allied soldiers were trapped on the Cotentin Peninsula, a breakout would be difficult.
And yet... Who would expect Normandy?
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How do you get large numbers of tanks and other war materiél onto the beaches? You can't just roll a 15-story-high merchant ship up onto the beach and expect it to disgorge its contents like a whale vomiting up Jonah. You need a harbor to provide protection for shipping from the heavy swells of the English Channel and Atlantic Ocean. You need piers which extend far out to sea, so that ships won't run aground.
Once your troops are on the beach, how do you get past the heavy defenses? Nazi Germany had been hard at work building the "Atlantic Wall" since March 1942. Anti-tank barriers had been erected along the Atlantic seaboard from the northernmost tip of Norway down to the Spanish border. More than 16 million land mines had been emplaced, and tens of thousands of sea mines. In Denmark, The Netherlands, Belgium, and France, large numbers of anti-aircraft batteries had been installed, and long-range artillery emplacements could reach far out to sea to keep large naval personnel and materiél transports away from shore. Anti-ship and anti-tank barriers were placed underwater and on the tidal flats. Pillboxes and machine-gun nests were placed every few hundred yards. Heavily fortified barracks containing hundreds of thousands of troops were set close by these emplacements. Punji sticks were planted in clearings and meadows -- any place that paratroops or gliders might land. Tens of thousands of miles of barbed wire had been laid. Any landing would come at high tide, so mines were placed at the high-tide mark. Tank turrets were scavenged and placed on the anti-tank barriers, their guns permanently aimed at the high-tide mark.
The first hurdle to be crossed was shipping. You can't invade if you don't have a harbor. Artificial harbors, however, were relatively easy to engineer. The Allies devised a simple method of creating them which proved immensely successful. The first element were "corn cobs" -- ships that crossed the English Channel under their own steam or were towed to France. These were then scuttled and filled with concrete. The "corn cobs" formed a long arc away from shore and out to sea. Once scuttled, they became known as "Gooseberries." Two "Gooseberries" (like two arms reaching toward one another) formed a "Mulberry", an artificial harbor. Floating, reinforced concrete caissons called "Phoenixes" were also towed across the Channel, sunk on either side of the Gooseberries, and filled with concrete. The Phoenixes helped anchor the "corn cobs" to the ocean floor. Now that the harbor was created, piers had to be created. "Spuds" were floating platform which formed the end of each pier. Designed to float, they also had legs to help stabilize the pier head in rough seas. Floating causeways named "Whales" connected the Spuds to land. A floating outer breakwater called a "Bombardon" was also emplaced outside each Mulberry to help lessen the impact of rough seas on the "corn cobs" and "Phoenixes."
The second problem was fuel: Getting fuel to the landing sites was going to be critical. Fuel simply could not be off-loaded from ships in the quantities needed. The best way to move fuel would be a pipeline. But how? You couldn't build a pipeline that floated, and laying one under the sea was impossible. Wasn't it?
The project to bring Allied oil to Europe became known as PLUTO -- the "Pipe-Line Under The Ocean." As early as May 1942, three months before the Dieppe Raid, the British were testing flexible undersea pipelines in the River Medway and the Firth of Clyde. It turned out that 3" (7.5cm) transatlantic telegraph cable sheathing worked just fine as a pipeline. It was big enough to pump large quantities of oil, but also flexible enough to simply be wrapped up on drums and then allowed to quickly unspool -- meaning almost instantaneous construction time! Several merchant ships were specially converted to carry the pipe on massive drums. Another test of the pipelaying, in very rough seas, occurred from December 26-30, 1942, across the Bristol Channel. Hundreds of oil pumping stations were built on the English coast to collect oil coming from refineries all over Great Britain. The pumping stations were housed in buildings designed to look like cottages and garages. A 1,000-mile network of pipelines was constructed at night to transport fuel from various British ports such as Liverpool and Bristol.
The third problem was the Atlantic Wall. And for this a host of ingenious devices were created:
- "Crab" tanks - A mine-clearing tank. The front of these tanks were mounted with a drum to which long chains were attached. As the drum turned, the chains would strike the ground and explode any mines before the tank reached it.
- "Crocodile" tanks - Flame-thrower tanks. The machine-gun was removed and a flame-throwing gun added. They had a range of 100 yards, were incredibly effective against bunkers, and were psychologically terrifying.
- AVREs - Armored Vehicles, Royal Engineers, these tanks had their main gun removed and a huge mortar attached. The mortar round was so large, it could take out a reinforced concrete bunker.
- "Bobbins" - Tanks which had their main gun removed and a huge mounted roller attached to the front. The "bobbin" in front would unroll heavy canvas or wooden dowels connected with wire. Once laid down, the canvas or dowels formed an improvised roadway over which other vehicles could drive without sinking.
- "Double Onion" - A tank with detachable mounted arms in front. The arms could carry massive explosive charges. The charges could be placed against a bunker's walls and set off remotely.
- "ARC" - Armored Ramp Carrier. A tank whose main gun was removed, and a large bridge-like structure on a mount placed in front. The bridge would be stored on top of the tank turret. When a ravine or stream was reached, the mount would swing up and forward, lowering the bridge (which could span 30 feet) over the obstacle. The bridge would be detached, and voila! -- instant bridge.
- Amphibious tanks - Tanks which could be submerged for limited periods of time. The goal was to let these tanks off miles from shore, and let them come onto the beach under their own power.
- "BARV" - Beach Armoured Recovery Vehicle. These were tanks whose turret had been removed and a large superstructure, including a grabbing/clutching arm, installed. They were used to drag damaged or sunk vehicles out of the way, or to refloat vehicles which had sunk into wet sand or water too deeply.
- Centaurs - Tanks with a bulldozer blade in front. Unlike the traditional armored bulldozer, the Centaurs could keep up with other fast-moving tanks.
- Canal Defense Lights - They weren't meant for canals, or for defense. The name was chosen to evade espionage. They were incredibly strong arc lights inside an armored casing, with just a slit which emitted light. The light was so strong that it blinded enemy troops at night, and could provide artificial daylight to illuminate targets.
- Bangalore torpedo - An explosive charge on a long, flexible pole. The charge could be placed against an anti-tank barrier or bunker wall and set off, while the men placing the charge were 30 to 40 feet away.
- Landing Vehicle, Tracked (LVT) - An armored amphibious landing vehicle which could carry 18 fully equipped men or nearly a ton of cargo. They were originally open-topped and designed only to carry cargo from ship to shore, and often broke down on hard land. But assault versions were also developed, which were fitted with turrets from Stuart light tanks. Bigger versions could hold eight, nine, or 11 tons of cargo, and some had fitted covers that enabled them to be completely submerged for a time.