Sidetrack for a moment.
The things that need to happen for Sealion to work:
- The Royal Navy needs to be tied up elsewhere. It doesn't need to be sunk per se, but just keeping it away from the channel is good enough for this.
- The RAF needs to become a potato, which is considerably more difficult. Had the Battle of Britain been won by Germany, though...
- Most importantly, Germany needs an economic policy that actually enables it to, you know, maintain a continued occupation of Britain. Since the WWII German economy was geared entirely towards conquering other stuff, this would have been difficult.
