Operation OVERLORD was probably the most complex military endeavor ever attempted. An amphibious landing against a defended beach is generally considered the most difficult kind of offensive operation. It’s so incredibly hard that it was considered impossible until 1921. Even so, it took the US Marine Corps and Navy almost two decades to work out how to pull it off. Even then, success wasn’t guaranteed. The enemy does have a vote, after all.
Any amphibious operation has several phases: the preparatory phase, which includes prelanding airstrikes and naval bombardment; the landing phase, in which the assault troops travel to and embark on the beach; the build-up phase, which sees reinforcements land in the beachhead; the expansion phase pushes the beachhead out, creating space for more build-up; and the breakout phase, during which the landing force pushes inland on the operational level.
Theater Differences
Every landing is different, of course. Most of the Pacific landings didn’t require a breakout phase. Small atolls like Tarawa and Kwajalein simply weren’t big enough. The expansion phase was enough. The much larger island of Guadalcanal was essentially an attritional defensive campaign fought along the American perimeter, and US forces never “broke out” operationally. The New Guinea Campaign was amphibious, but it was aimed at establishing coastal enclaves to support the movement to the Philippines. Only the larger Philippine islands, like Luzon, encompassed the entire spectrum. Even bloody Okinawa saw an unopposed landing, resulting in a long, slogging expansion with no actual breakout.
But Europe was different. Unlike most of the Pacific operations, European landings were aimed at continental targets. Where the Pacific landings had limited objectives, even in the Philippines, European operations were merely the first step in projecting continental-level power, meaning the build-up and expansion phases were much more logistically intensive. That doesn’t mean the Pacific landings were easier. Ask any Marine who hit Iwo Jima or Peleliu. They just had different objectives, and the enemy’s means of counterattack and reinforcement weren’t the same.
Pacific landing forces generally dealt with the Japanese forces that were already in place, which was bad enough, given the Japanese tendency to fight to the death. However, the Japanese had to run the US Navy’s gauntlet to bring reinforcements, which they were rarely able to do after 1942.
The Germans, however, could potentially bring ground and air units from all over the continent to bear on any European landing. And no US aircraft carriers and submarines blocked their path. Any force directed toward those German reinforcements had to project from the beachhead or its supporting elements, including the airborne divisions on D-Day. Only the Allied tactical air units could be seen as interdiction forces roughly comparable with the Navy’s Pacific task forces. Nevertheless, Germany could move land forces to Normandy or Salerno far easier than Japan could reinforce across the vast Pacific.
Momentum, Momentum, Momentum
Like any attack, an amphibious assault depends on momentum. But if a conventional ground assault fails, the attackers can at least fall back on their own lines. Amphibious troops don’t really have that option. So, momentum across the beach must be established quickly and constantly maintained. Failing to create that momentum through efficient ship-to-shore movement means that reinforcements and supplies stack up on the beach, eventually clogging it so nothing else can land. Momentum allows those reinforcements and supplies to move along, making space for more men and materiel. Omaha Beach on D-Day is a prime example of not establishing momentum.
Creating momentum relies upon several factors: effective fire support to establish local fire superiority; properly timed waves and sub-waves to maintain flow without stacking up or creating gaps; proper organization of waves to enhance unit cohesion; and effective processes for facilitating the flow of materiel off the beach. If any one of these factors fails, the entire enterprise can, and probably will, collapse.
Fire Support and Ship-to-Shore Movement
Waves and sub-waves must be organized to land the assault troops at the right place at the right time. That’s harder than you might think, given the influence of weather, currents, visibility, beach obstacles, and enemy fire. It didn’t help that the Normandy landing craft line of departure was 11 miles from the American beaches. The initial waves for Omaha and Utah Beaches drifted too far east and south, respectively, thanks to inadequate boat crew training and strong currents. The Utah landing force was further hampered by smoke on the beaches from naval gunfire. The boat drivers couldn’t make out their landmarks.
The result was that the first wave on Utah started the war 2,000 yards south of where they were supposed to. But the Omaha first waves got bunched up on the beach, making them vulnerable to enfilade fire from German defenders that had nothing in front of them. Omaha’s first waves were a classic bottleneck, causing the follow-on waves to stack up. Omaha was also probably the single best-defended stretch of beach on the planet, which didn’t help matters. Finally, low cloud cover caused the pre-landing airstrike by 480 B-24 bombers to miss the beach entirely. Only direct naval gunfire on German emplacements and fortified beach exits saved the day at Omaha when almost everything else broke down, which is a story unto itself.
The problems on Omaha had many causes, about which entire books have been written. I recommend Omaha Beach: A Flawed Victory by Adrian R. Lewis.
Unit Organization
The Marines learned in the 1930s that units spread out in skirmish lines across a landing beach get bogged down quickly. Wider fronts are harder to control, and they have no depth, so reinforcement must necessarily come from another unit, creating an extra level of communication. The Marines fixed this problem by narrowing unit fronts, whether they be company, battalion, regiment, or division. This allowed commanders to more easily control their men. They also had their own reserve to maintain any momentum gained. They could reinforce on the spot instead of relying on another unit. The Marines described this as “small groups of skirmishers, arranged in chains of groups” and likened it to infiltration tactics, ideally with armor and direct supporting fire.
So, a beach that could theoretically be covered by a division was instead divided into two division areas. This increased each division’s depth while reducing its front, giving each division commander better control and more options. This effect carried all the way down the organizational chart to individual companies.
That’s why two divisions, the 1st and 29th Infantry, landed on Omaha Beach, even though it was technically a one-division front. The other Normandy beaches had much easier egress opportunities and so only had one assault division each. The Normandy planners knew Omaha would be a tough nut, so they opted for the split front’s greater depth and control. It also provided the opportunity to land the veteran 1st Infantry Division alongside the green 29th. Why the 29th made its combat debut in Omaha’s first wave is another question entirely. But the Big Red One’s presence was the other saving grace on Omaha.
Shore Management
Moving materiel off the beach and getting it to the combat troops requires more than you might think. Early amphibious exercises saw logisticians loading transport ships like bean counters. They maximized the available space and weight allowance. That resulted in one exercise where no food or fresh water came ashore until the second day, but a complete post-exchange was set up within a few hours. Medical supplies did not arrive until the 9th day because they were packed at the bottom of the ship’s hold. This was a simulated combat landing, and the deficiencies were clear.
The Navy got the message and developed a system called “combat loading.” Combat loading ignored wasted space but configured ship loadouts so that the most important items, like ammunition, food, and medical supplies, were unloaded first. That included heavy weapons since exercises showed that the assault troops should be lightly encumbered.
The Navy and Marines developed specially trained shore parties, commanded by Beachmasters, to control the flow once ashore. These were in place by 1940. The Army understandably had other priorities than amphibious warfare during the 1930s. Only when France fell in 1940 did it dawn on the Army and the British that amphibious capability would be necessary to re-enter the European Continent. Army shore parties learned their trade in the 1942-43 Mediterranean campaigns.
Combat loading and shore management had been perfected by the time Normandy arrived. But another important factor in amphibious operations—lift capacity—almost compromised them.
The Problem of Lift
“Lift,” in this case, refers to how many men and how much stuff can be accommodated on the ships and boats available. You can have all the men and weapons in the world, but they’re useless if you can’t effectively carry them to the beach. And the assault waves must be ready to fight when they land. They won’t have time for ammo and other necessities to land later, meaning that proper combat loading is critical.
Exercises in 1935-36 demonstrated that the landing force must be equipped with enough boats to embark the assault force simultaneously, including the reserve. Momentum can only be maintained through the continuous landing of assault waves, which are not subject to delays in embarking follow-up forces or bottlenecks caused by boat attrition. They also showed that the only way to materially affect the ground action on the beach during the landing phase was to commit the reserve force in a timely manner. If lift for the entire assault force could not be accomplished, the initial wave was subject to defeat in detail.
The OVERLORD plan called for landing the equivalent of five infantry divisions, four armored brigades, five self-propelled artillery regiments, shore control units, air force and naval spotters and liaisons, and an immediate follow-on wave of two-thirds of a division. All those units had to be loaded on small landing craft before dawn on June 6. The rest of the follow-up units, one and one-third infantry divisions, were loaded on transport ships to be landed on June 8. They used the same landing craft employed by the assault waves.
The landing craft were in particularly short supply. Training bases and repair yards were combed to provide every craft possible. Minimum acceptable serviceability rates were raised from 85% to 90 and 95%, depending on type. Operation ANVIL, the planned simultaneous invasion of southern France, was finally canceled due to inadequate numbers of landing craft to accommodate both operations simultaneously. It didn’t help that significant numbers of landing craft hulls were needed for close fire support, carrying guns, artillery, and rockets.
Supreme Allied Commander Dwight D. Eisenhower, attempting to preserve ANVIL, pressured planners to pack their ships and craft tighter, thus compromising proven combat loading techniques. Eisenhower’s combat commanders protested, and Ike was overruled by the Combined Chiefs of Staff. Once ANVIL was canceled, however, sufficient craft became available. Even so, the OVERLORD logisticians maximized their lift and space within established combat loading parameters, even pre-packing trucks before loading them on transport ships.
4,126 landing craft of all types were used at Normandy. Every single one was needed. Maintenance crews stepped up, delivering serviceability rates of above 99% for American craft and above 96% for the British. In the end, logistics, in the form of lift, was the primary determinant of the size and scope of OVERLORD.
Odds and Ends
A few other interesting tidbits about Operation OVERLORD:
The Influence of Dieppe
The Allies achieved operational and tactical surprise at Normandy, partially because the Germans believed the main thrust would be at a port city, namely Calais. This belief stemmed from the August 1942 Dieppe Raid, in which Canadian forces raided the French port city to test German defenses and gauge the possibility of actually taking a port. The raid was a disaster for the Allies. The Canadians were repelled with heavy casualties.
The Germans grasped the raid’s purpose but drew the wrong conclusions about its results. They saw it as proof that the Allies intended to attack a port city so they could efficiently unload transport ships to supply the landing force. They assumed the Allies would take the lessons from Dieppe and be better prepared in 1944. The Allied deception plan, Operation FORTITUDE, reinforced this belief.
In fact, the Allies took the opposite lesson. Dieppe proved to Allied commanders that German-held port cities were too well-defended to attack directly. Coastal artillery batteries were even more powerful than heavy naval guns, making Nelson’s adage that “a ship’s a fool to fight a fort” still valid. Port cities were also well-garrisoned with troops fighting on their supply base, while amphibious forces were always the weakest on the front end. Assaulting a port just didn’t make sense. The November 1942 Allied assaults on the North African ports of Oran and Algiers reinforced this view. Dieppe caused the Allies to steer clear of attacking a port from the sea.
The Mulberry Harbors
The lessons from Dieppe notwithstanding, the Allies still needed port facilities to ensure a reliable build-up leading to the eventual breakout. When the need for a port to support the build-up phase came up in conversation among the planning staff, British Commodore John Hughes-Hallett commented, “Well, all I can say is, if we can’t capture a port, we must take one with us.” He brought in a plan for just such a project the next day.
These artificial harbors were code-named “Mulberry.” They were built from pre-cast reinforced concrete caissons and sunken ships. Even as the assault troops pushed off the beach, Allied ships were placing the 6,000-ton concrete breakwaters, with the gaps filled by ships towed into place and sunk. The caissons supported unloading piers, and pontoon roadways connected them to each other and the beach. Each Mulberry Harbor was about the same size and capacity as Britain’s Dover Harbor.
Two Mulberries were built, one for American use, the other for the British. Mulberry (A) was destroyed off Omaha Beach by a storm on June 16, but Mulberry (B), off the British Gold Beach, remained operational for six months, after which it was no longer needed. The Mulberries decreased the unloading time for the workhorse Landing Ship, Tank (LST) by over 90% as opposed to the time required to unload on the beach. LSTs carried all kinds of cargo, despite the name, though they could indeed carry tanks directly to the beach.
Finally, the Allies constructed and quickly laid a cross-channel undersea pipeline to carry gasoline directly from England to Normandy. Pipeline delivery proved easier, faster, and safer than loading gasoline on board tankers and then unloading it at the Mulberries.
Duplex Drive Tanks
The Allies had long desired an amphibious tank to support landing operations. The US Marines tested one as early as 1924, but nothing suitable was ever developed. The Allies tried again for Normandy, modifying existing M4 Sherman tanks. The Duplex Drive (DD) Tank featured two propeller shafts for waterborne movement (hence the name) and a canvas skirt to make them float.
Tests in calm, protected waters were promising, but the DD tanks were not built for rough seas. 27 of the 29 tanks assigned to Omaha Beach’s first wave sank in the rough water. The other beaches had a better success rate, but the lack of armor support contributed to the early chaos on Omaha.
Air Superiority
Air superiority, preferably air supremacy, is a basic prerequisite for a successful amphibious landing. Landing ships subject to constant air attack are unlikely to effectively put their troops and supplies ashore, after all. The German Luftwaffe made but a brief appearance over the Normandy beachhead on D-Day and was rarely seen during the build-up.
The Allied strategic air forces had deliberately attacked targets that the Germans had no choice but to defend during the months before OVERLORD. A primary goal of those raids was to lure out the German fighter planes and destroy them. This was made possible by the long-range escort capabilities of the P-51 Mustang fighter. Allied tactical aircraft also targeted Luftwaffe airfields and facilities in France, eventually forcing the Germans to pull them back out of range.
These efforts combined to make the Luftwaffe a non-factor in the German coastal defenses. Landing the troops was hard enough, but the Allied ships and troops didn’t have to worry about being attacked from the air. Air supremacy also allowed Allied planes to spot naval and conventional artillery fire. Aerial spotting was crucial for maximizing those assets.
The Airborne Assault
The Allied airborne mission is covered by a separate article in this series.
Deception
The Allies’ elaborate deception operations supporting OVERLORD are covered in this series’ article on Normandy’s strategic aspects, as is the Allied advantage in weather forecasting.
Final Thoughts
Even this far in, we’ve barely scratched the surface of OVERLORD’s complexity. Entire books have been written about this stuff, many focusing on only a single aspect. The literature is simply enormous. But I’ve tried to present a glimpse of how the Normandy landings worked and the principles on which they were planned and executed.
The United States Joint Chiefs of Staff originally wanted to launch an OVERLORD-style invasion in 1942. That idea lost out to Operation TORCH, the invasion of North Africa. The Chiefs then pushed for a 1943 landing, which was overruled in favor of the Mediterranean campaigns in Sicily and Italy.
Given the OVERLORD plan’s intricacies, it’s difficult to see how the Allies could have successfully invaded Northern France before 1944. OVERLORD planners drew on lessons learned from Dieppe, North Africa, Sicily, Salerno, Anzio, and, to a lesser extent, the Pacific. Those experiences did not exist in 1942 and were ongoing in 1943.
Furthermore, OVERLORD’s massive logistical requirements could not be met in 1942, though they could have, arguably, been available in 1943. Operation Husky, the July 1943 invasion of Sicily, was larger than OVERLORD in terms of men and units employed. But HUSKY’s operational and strategic goals were far more limited than OVERLORD’s, meaning the logistical situation was very different. HUSKY was launched to liberate the island of Sicily. A large island, to be sure, but nothing compared to the European continent. OVERLORD required far more depth than HUSKY.
OVERLORD was the ultimate amphibious test. The Allies passed with flying colors despite a few significant problems. But to paraphrase German Field Marshal Helmuth von Moltke the Elder, no plan survives first contact with the enemy. The OVERLORD planners did their work well, and the contingencies, like direct naval gunfire on Omaha Beach, were enough to overcome the setbacks.
The difficulty of planning and executing an operation like OVERLORD cannot be overstated. Especially considering the extent of operations in other theaters. The United States, Great Britain, and Canada launched OVERLORD while simultaneously running full-scale operations in Italy. Rome fell the day before Normandy was invaded. US forces were also heavily engaged in the Southwest Pacific and executed the three-division amphibious invasion of Saipan nine days after OVERLORD. The resulting Battle of the Philippine Sea was one of the largest naval engagements in history. The scope staggers the imagination. US power became truly global in mid-1944.
Today is Operation OVERLORD’s 80th anniversary. 80 years ago today, the Western Allies took the first step toward Germany’s final defeat. But OVERLORD’s success was literally years, even decades, in the making. Without that work, the Normandy invasion would have been far more difficult and may not have happened at all.
Dwight D. Eisenhower brought some perspective to that notion in an interview with historian Stephen Ambrose. Eisenhower asked Ambrose whether he had ever met Andrew Jackson Higgins, the inventor of the Higgins LCVP, the ubiquitous small landing craft of World War II. Ambrose said that he had not. “That’s too bad,” Eisenhower replied. “He is the man who won the war for us. If Higgins had not designed and built those LCVPs, we never could have landed over an open beach. The whole strategy of the war would have been different.” The same could probably be said of many who contributed behind the scenes to history’s most complex military operation.
