How strong a land force did Germany need for a conquest of the British Isles in the summer of 1940?

Question

In 1942, America's General Eisenhower summed up his gloomy assessment of an invasion of Nazi-held Europe in the "reverse" direction as 50-50 to get one division ashore, and one chance in five (20 per cent) to maintain a five division bridgehead. While this may have been a "guess," it certainly qualified as an "expert" opinion.

I'm going to assume that the German naval and air forces would play their roles as described below.

The German navy feared that it could get only "so many" men across the English Channel before losing every ship. Then the air force would have to re-supply and reinforce the landed men (in the manner of Crete) before the Germans could finally accumulate enough men to overwhelm the British army and occupy the British Isles:

What did either contemporary German or British strategists or planners, or recognized historians in hindsight, have to say about any of the following questions:

1. How large a force X, was necessary to establish and maintain a beachhead that could only be resupplied and reinforced by air?

2. How large an additional force, Y, would be necessary to complete the occupation? Put another way, what was the necessary sum of X+Y to defeat the British army?

3. How strong were the British land forces available in Britain in the summer of 1940?

Answer 1

Sea Lion had little chance of working. Everyone underestimated the complexity of a large scale amphibious invasion in 1940. The Germans considered Sea Lion to be like a river crossing on a broad front. The Americans were pushing for an invasion of Europe as early as 1942. The Allies had the luxury of learning the hard way through invasions of increasing scale from North Africa to Sicily to Italy before undertaking Overlord. Even then it barely worked out. Curiously, there was little liaison between the US forces in the Pacific and the invasion forces in Europe.

The Germans had no large scale amphibious experience, they didn't stand a chance.

You're asking three big questions. I'm going to focus on one, what could be supplied from the air, because the answer renders the other two moot.

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How strong were the British land forces available in Britain in the summer of 1940?

Here is the British order of battle 11 September, 1940, though that's just units on paper.

550,000 experienced soldiers were evacuated from France, but lost most of their heavy equipment. In addition, about 100,000 French soldiers were sent back to fight in France and were lost leaving about 450,000 in Britain. On top of that, Britain had a home guard of questionable use.

I'm not going to go into this much further, and you'll see why in a moment.

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How large an additional force, Y, would be necessary to complete the occupation? Put another way, what was the necessary sum of X+Y to defeat the British army?

A lot. Here is the German order of battle for Operation Sealion. I count 28 divisions, on the scale of the 6th Army at Stalingrad.

They planned on 11 infantry divisions plus 2 airborne in the first wave, 8 Panzer and motorized divisions in the second wave, and six infantry divisions in the third.

Would this be enough? Its within the ballpark of what the Allies found necessary in Normandy, and the Germans were good at planning land campaigns. It's a big question and I'm not going to get into detail because the answer to the final question renders it all moot.

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How large a force X, was necessary to establish and maintain a beachhead that could only be resupplied and reinforced by air?

Let's break that down into two questions. First...

How large a force could the Germans resupply and reinforce by air?

Germany failed to achieve air superiority over Britain in 1940 and never had a prayer of naval superiority. It's questionable how much of their invasion force would even reach the shore. Resupplying their beachhead is going to get ugly.

I'll use the attempt to resupply the 6th Army at Stalingrad as a yardstick. They were attempting to supply about 20 divisions totaling 210,000 men behind enemy lines while under air attack.

German strength in the pocket was about 210,000 according to strength breakdowns of the 20 field divisions (average size 9,000) and 100 battalion sized units of the Sixth Army on 19 November 1942.

They calculated they needed a minimum of 300 to 700 tons per day. That is to maintain a defensive perimeter with enough fuel for counter attacks. Maintaining offensive operations would require far more, I'm going to guess 1000 tons.

By pressing every available aircraft into transport service, the Luftwaffe was able to transport about 85 tons per day. The best they ever achieved on a single day was about 250 tons. They lost about 500 aircraft in the operation, half their total committed capacity. The 6th Army starved.

The Luftwaffe was able to deliver an average of 85 t (94 short tons) of supplies per day out of an air transport capacity of 106 t (117 short tons) per day. The most successful day, 19 December, delivered 262 t (289 short tons) of supplies in 154 flights.

Let's get some rough numbers. 1000 Luftwaffe aircraft delivered 85 tons under fire or .085 tons per aircraft. 200,000 men require about 1000 tons of supplies per day for offensive action (rough guess), or 0.005 tons per man.

That means 1000 Luftwaffe aircraft, their best effort could supply about 17,000 men or about two divisions.

What about Crete?

The Battle of Crete lasted just 11 days over a relatively small area using 30,000 airborne and mountain light infantry. Easier to transport and supply using the 500 transports at hand. The Germans had air superiority and captured an airfield early. Even so, they lost 370 aircraft in just 11 days.

The British forces were about equal in number and were operating far from their home island and had the option to retreat and evacuate, which they did. Had they hung on another week, the German supply position and loss of transport aircraft would have become dire.

But 500 transports for 30,000 1940s light infantry is in the ballpark of the 1000 transports for 17,000 men of two 1942 mechanized divisions.

Now the second question.

Is this force large enough to hold a beachhead?

Not a chance.

Even with the dilapidated state of the British Army, two divisions did not stand a chance against half a million British. Remember, this is the German army of 1940 fighting with Panzer Is, IIs and a few III and IVs. Two divisions is not just the beachhead, it is the maximum invasion force they can sustain. All while steadily losing transport aircraft to the RAF.

In Overlord, the Allies barely held on with their initial eight divisions plus naval and air dominance. It took a build up of 39 divisions, a million troops, to break out of Normandy.

Even if the RAF was destroyed, there is no hope for Germany to supply an army large enough to defeat the British army and get off the beaches.

Answer 2

Since 1588 the British Home Fleet has had a single reason for existence - defence of the home islands. Comprising in June 1940 a total, with allied vessels from the Royal Dutch and Norwegian Navies and omitting vessels specialized for convoy, anti-submarine and minesweeping patrol, of

• 4 battleships: Barham, Nelson, Rodney, Valiant
• 3 Battlecruisers: Renown, Repulse & Hood
• 2 aircraft carriers: Ark Royal & Furious
• 7 heavy cruisers: Berwick, Devonshire, Norfolk, Suffolk, Sussex, Birmingham & York
• 10 light cruisers: Aurora, Penelope, Cardiff, Galatea, Edinburgh, Glasgow, Newcastle, Southampton, Manchester, & Sheffield
• 37 destroyers (including 3 Dutch & Norwegian):

In order to prevent elements of this force from repeatedly sweeping though the channel destroying everything afloat in its way, the Germans would have had to not only achieve air superiority, but maintain it in the face of a determined RAF equipped with superior planes (Supermairine Spitfire, comparable in every way with the ME 109 but with almost twice the operating range) for the entire duration of the battle.

The notion that the Germans could land a force in England capable of subduing the 338,000 veterans evacuated from Dunkirk reinforced by another 200,000 odd Commonwealth troops, and keep it supplied long enough to do so was even more ludicrous than Goring's claim of being able to supply Van Paulus by air.

Answer 3

The idea behind this question seems to be to better understand what it would have required for the Germans to have invaded England in WW2. How close were they to that objective?

To conduct an invasion like this, there are two main factors: (1) tactical (can your weapons beat their weapons) and (2) logistical (can we move across enough stuff?). Your question focuses on item (2), how much stuff do we need?

Unfortunately for the Germans, they never really got to step (2). Let's consider problem #1. If you solve that problem, problem #2 is actually pretty easy. With enough tactical domination you can move guys across in canoes if you have to. The real issue is the tactical problem.

From a tactical point of view, the British had 3 capital ships and a bunch of destroyers, like 20 or something like that, defending the channel. To invade you would need to destroy those ships or drive them away. The British constantly patrolled the channel with Spitfire fighters looking for enemy ships and planes. Even a single battleship can absolutely annihilate any number of troop carriers, and it is much faster than a troop carrier or logistics vessel. Even a single destroyer with just four 5" inch guns could just tear apart any invasion force. It would be sickening to watch that. Imagine 20 destroyers and a couple of battleships. Unthinkable.

To neutralize these ships, the Germans needed to get air control. With air control, they could use dive bombers like Stukas to destroy the ships or confine them to harbor. They failed to get this air control in the Battle of Britain. If they had succeeded in destroying all the British Spitfires and other fighters or forced them to stay grounded, then the British would have had to move their ships out (or watch them get destroyed). The inability of the Luftwaffe to do this meant an invasion was impossible.

What if Air Control Had Been Achieved?

Now, to get to your main questions, if air control had been achieved, what kind of land army would be needed? The British home army had something like 20 divisions and 1000 guns. By comparison the Germans used about 100 divisions just to invade Poland alone, including 9 armored divisions. If they had landed a holding force of 5 divisions with enough artillery battalions, about 50,000 men (including logistics) and 500 or so guns, that would have been enough to establish a base to import the rest of the army, which would of course have been completely decisive. Nevertheless, getting 50,000 men across the channel would have been a non-trivial exercise. You would need hundreds of landing craft plus logistics vessels to provide food, water, ammo, and fuel.

These were all solvable problems, though. The problem that did not get solved was the war in the air.