When anyone mentions alternate history, there's a good chance the first thing that pops into your head is a Europe where the Nazis were victorious.
There's good reason for that; several sterling works of fiction such as Robert Harris' Fatherland and Len Deighton's SS-GB work from that assumption. Something both those tales have in common however is the absence of a comprehensive explanation as to how that victory occurred.
When I first dabbled in AH, I headed over to alternatehistory.com to see what people thought about my ideas to make Operation Sealion successful. The first thing I learned rather quickly was that this particular trope seems fairly well-hated in the AH community; I was told to never again refer to the 'dreaded sea mammal'!
But what about these great stories that assume a Nazi invasion of Britain? Millions of readers can't be wrong?
A kinder observer reminded me that a novel is my story; it's my fiction and I should do with it what I will to tell the narrative as I imagine it. Sound advice I thought. But then I learned from more educated colleagues the military reality of the situation in 1940, and discovered why it is an unpopular premise amongst AH hard-liners - for fifty Wehrmacht divisions to successfully land in England, you have to ignore pretty much every real-world constraint of battle planning. They figured this out in fact at Sandhurst in 1974, with the help of some Germans who knew what they were talking about: https://en.wikipedia.org/wiki/Operation_Sea_Lion_(wargame)
Was I downhearted? No. I was brought alive to a possibility that would satisfy my fiction and also my own desire for an accurate military reality: an Operation Sealion that was launched, and ultimately failed.
As I continue to explore this idea in my sequel to The Eagle and the Empire, I shall proudly proclaim when asked that I am not afraid to write about a German invasion of the United Kingdom! After all, I never claimed it would work out for the Nazis...and who wants it to?