The only major issue with Operation Sealion was Germany's lack of available transport ships. The British obviously had a great natural advantage for themselves with the presence of the Channel, and it would have obviously taken a while for Germany to even build up the forces in reserve to transport all that stuff. The amount of labor it would have taken to secure the Channel, especially considering Germany's real lack of naval superiority, would have been astronomical. They would have had to completely secure all of coastal France, when they were already dealing with a large resistance movement in the area. Then they would have had to utterly destroy the Royal Air Force, since their ships were still close enough to cause serious damage to any ships trying to do anything in the Channel/Northern France... nevermind that they'd have to clear the Channel of the kajillions of mines that the Brits always put down in case of a major issue with the rest of Europe, and they'd have to have put up their own to secure an area that they now officially owned... Then, of course, they'd have to make sure that every section of the British land forces were destroyed enough to allow them to actually initiate the basic fast-moving principles of Blitzkrieg, which revolves around a surprise attack that gives you offensive advantage and a large amount of speed that would allow you to just walk past anyone in your path.

Of course, by the time Germany even started to consider Operation Sealion, Britian was the most powerful and accessible foe they had... and they were prepared. They had invented radar to help them spot incoming enemies of any kind, they had obviously mined up the channel to high heaven, had seriously increased their fighter plane output, and had basically prepared for siege warfare along the channel. Obviously, when you've got an enemy ready for you like that and there's no way you can get to them by land, you can't really use any of the Blitzkrieg tactics that your soldiers are trained in.

The mere fact that the channel exists is what has kept Britiain sovereign and so powerful for so many years. Any time any issue sprung up on the continent, it took an extra generation or two for any actual effects to be felt on the islands.

The French Navy would have been large enough to possibly have really attacked England (before the British were smart enough to blow it all up), but the Germans had never developed a real navy like that and could barely even manage to hang onto any of the areas they got across the ocean - Erwin Rommel's fighting in Libya shows just how little oceanic logistics the Germans possessed or cared about, and also just how strong they were even when isolated like that.

Now, add all of these facts up, and a German invasion of Britain would never have worked at the time they were planning in the early 40's. :/ Germany just was not prepared to even consider attacking Britain with their own material - their plan was clearly to scrape together conquered navies and use those, but British bombers took care of those and the organized French resistance did in the final remnants during Operation Lila in '42.

If Germany could have taken down some more British bombers before they took out most of the former French Navy during Operation Catapult and had done Operation Lila earlier before it wouldn't have worked, and then had quickly acted on Operation Sealion before the Britiains could increase their own wartime production levels, they would have had a chance of being able to take over Britain. But obviously, once their only real landable naval forces were gone (and even then they were all parasite forces, far weaker and more poorly-trained than the professional Royal Navy men in Britain), their chances were about 0.

Of course, if it had worked, the Germans would have won the war in Europe right there. D-Day would have never happened, since the US wouldn't have had a strongly-allied and well-built jumping point to launch from anywhere near the European continent... Basically, the only real German threat from that point on, in my opinion, would have been from Russia. And at that point, the Germans had a non-aggression pact with them and the Russians really weren't well built-up at all. If a Russian war had started and they had started marching towards Moscow in the early Spring when they didn't have two fronts to fight on, I'm sure they would have made it through and taken over what parts of that area they actually wanted.

:::shrugs:::


tl;dr - Germany wasn't prepared for a swift naval crossing or any British defense and so Operation Sealion would never have worked. If Germany had, though, it would have been totally awesome for them and Europe would still be called the Unhappy Subjects of Awesome Germany. :3

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