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Operation Sea Lion – the Unmentionable Sea Mammal

Updated: Jan 23, 2019

By Andy Cooke


It's fascinated alternate historians, soldiers, pilots, and sailors for eight decades. It even gave its name to this very publishing house. But why was it doomed to fail?


Unternehmen Seelöwe (Operation Sealion) was the codename for Nazi Germany’s planned invasion of the UK in 1940. The idea was that the Wehrmacht would be conveyed across the channel in barges, escorted by the Kriegsmarine (who would block the channel from the Royal Navy for the duration of the crossing) while the Luftwaffe obtained total air superiority – the latter two being essential, otherwise the phrase “shooting ducks in a barrel” would have been literally appropriate.


It’s a favourite of alternate historians as a Point of Departure (PoD): What If Sealion had occurred and been successful? (with the answer – “let’s write about a Nazi domination of the UK!”)


It’s caused so many arguments (on the line of “Ah, no, it would never work” by serious researchers versus “Come on, I really want to write that story, surely there’s some way it could work?”) in alternate history circles that it’s been called “The Unmentionable Sea Mammal” by alternate historians sick and tired of quashing it. And yes – this is the inspiration for the name of Sealion Press – a huge majority of alternate historians will recognise the name as being integral to alternate history controversies.


“So why couldn’t it have succeeded?”

By the time the plan was seriously proposed, Germany had no chance of having enough resources in theatre. The Luftwaffe-vs-RAF was a challenge to start with; the Kriegsmarine-vs-RN would have been ludicrous.


“Ah, but the U-boats could have blocked the Channel!”

U-boats need room to manoeuvre. Stuck in the Channel (in shallow confined waters), in daytime, in good weather with good visibility (because anything other than that would simply be the Nazi’s suiciding all their attackers) would lead to Happy Hour for the Royal Navy and RAF, and the end of the German U-boat fleet. U-boats had as their main weapon the fact that no-one knew where they were. Stacked side-by-side across the narrow Channel and it’s “Good night, Dönitz”. (There was a plan to block one end of the Channel with mines and 14 torpedo boats – facing a couple of dozen destroyers. You have to wonder how optimistic the torpedo boat captains would be in this scenario).

In theatre, the Kriegsmarine had 1 capital ship (vs 5 for the RN), 1 cruiser (vs 11 for the RN), 10 destroyers (vs 53 for the RN) and 20-30 U-boats (aka “floating coffins” if used in the way envisaged). The Kriegsmarine were going to bring in over a thousand river barges – boats designed for the Rhine, with shallow freeboard, that would sink in anything above “Calm” sea conditions (The wave from the wash of a fast destroyer would sink one. You have to feel sorry for any poor infantrymen forced onto these floating deathtraps).


After attempting to block the Channel with doomed U-boats and suicidal torpedo boats, the Kriegsmarine were planning on breaking into the Atlantic and hope the Home Fleet would happen to abandon their posts and obediently follow them out there.

Then these unsafe barges, loaded to the max with soldiers (but undermanned with actual sailors to control them) would be towed (yes, towed) across the channel at 2-3 knots. Taking 30-50 hours, depending on tides, to carry out an opposed amphibious landing.


“Couldn’t the Lufwaffe have taken out the RN?”

No. Bear in mind that (with command of the air for long periods), the Luftwaffe managed to put 4 out of 39 RN destroyers out of commission at Dunkirk. In a small harbour, often while standing put embarking troops.


“What if the Luftwaffe had continued attacking 11 Group’s airfields and driven them out of South England?”

Unfortunately for the Nazis, 11 Group’s backup plan (if it lost its airfields in Southern England) was to fall back to the Midlands, out of range of the Luftwaffe - and continue fighting from there. And then 10 Group and 12 Group would join in as well.


The infamous “Big Wing” philosophy espoused by 12 Group – to mass fighters in the air first before proceeding to intercept the Luftwaffe with overwhelming firepower – was a poor one for the Battle of Britain (as the Luftwaffe tended not to hang around for the RAF to mass carefully and dress ranks and were usually only intercepted, if at all, on the way home after having actually done their mission), but would have been frighteningly effective on an enemy proceeding at walking pace in good visibility at sea level for more than a day. Bear in mind that the combined RAF forces only need one good attack (in 30+ hours) to end the invasion – assuming they get to the targets before the Royal Navy, who (as long as they didn’t obediently toodle off to somewhere else, as fantasised by the Nazi planners) would have had overwhelming naval superiority.


No - those 30-50 hours would have been in the face of relentless air assault – for those barges that hadn’t already sunk due to some low waves (of which the Channel does have quite a few), passing destroyers that somehow got past the doomed U-boats and didn’t decide to wander out into the Atlantic after all, and the occasional outright crashing into other poorly controlled barges and tugs.


Now, assuming your author has somehow managed to get past all this, you’ve magically landed 9 divisions (for such was the plan and the highest number that could even be imagined by the plans the Nazis had) on the south coast of England. You can’t resupply them in under 8-10 days. You don’t have heavy equipment. You’re facing a dug-in enemy (see “Martello Towers” as just one example) operating on its own ground, with short resupply lines, its own artillery and heavy equipment readily to hand, and 28 divisions against your nine. Unless your army consists of battalions of supervillains, you’re not making it to that resupply date.


We haven’t even got to the absurd resupply plans (involving paratroopers landing a dozen miles from Dover in hills, with no escort from the Luftwaffe) and no tactical plans, but still expected to capture the heavily defended and fortified port intact on their own and without assistance.


As it happens, the few exercises and practices carried out by the Nazi forces in preparation for this ranged in outcome from “chaos” to “fiasco”, as the various branches of the Nazi military may have heard of “close co-operation” between the branches of the military, but certainly didn’t seem to like the concept; each branch expecting the impossible from all the others without ever bothering to do such trivialities as plan or practice together.


So – the Germans didn’t have suitable transport for their forces to cross the Channel. Neither did they have the ability to fight past the RN and RAF. Or, in fact, the sea itself, given the transport barges available. Beyond that, they didn’t have any faint possibility of having enough forces on the ground in the South of England to survive for more than long enough to say, “Oops, here come the Allies and boy, they’re pissed off.” The fact that resupply plans were ephemeral and implausible is simply the icing on the cake.

No, there’s a reason Sealion is known as “The Unmentionable Sea Mammal,” in alternate history circles. There’s simply no plausible way to make it work. You’d have to wave a magic wand and have mystic portals appear between France and South England (Hmm, that’s an idea. Excuse me while I go and make some notes).


A more usable PoD would be: What if the Nazis had gone ahead with Sealion in the face of these problems? After all, the Nazi High Command weren’t loathe to make some really irrational decisions. What would have happened if, in 1940, the Germans lost nine divisions in the Channel, most or all of their existing U-boat fleet (together with all experienced U-boat commanders), suffered crippling damage to the Kriegsmarine, and a massive psychological defeat?


The above article was based on work by Alison Brooks and David Flin. The article also drew upon discussions on alternatehistory.com and the author’s own studies at the Defence Academy of the United Kingdom.


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