@VeniVidiWeeWee
In modern times 1941, Battle of Britain.
Without that the world would be very different.
I've always believed that the Battle of Britain really isn't the defining moment the BBC One documentaries and such portray it as.
The narrative is often that had the RAF been defeated that German invasion and conquest would have automatically followed. This is hugely over-simplistic and totally ignores the reality of the practicalities involved.
Nazi Germany, for all it's conquests, never at any point launched an amphibious assault on the sort of scale that would have been required to effect a sustainable landing in the UK. If, a big if, the RAF had essentially been knocked out of the sky, Germany still faced the prospect of encountering the Home Fleet in the English Channel, which they had no capability whatsoever to match in a straight fight. Air superiority would have helped, but they still would not have been able to prevent the Royal Navy completely obliterating any attempt at a crossing of the Channel.
On top of this slight hinderance, Germany did not have the means to actually launch an amphibious crossing of the channel. They were having to commandeer river barges and coastal tugs as it was just to maintain their supply chain. They did not have the equipment necessary, as well as having absolutely no experience of mass amphibious assault, nor the developed doctrines the experience would provide. Even the conquest of Norway required mass transit of Nazi troops and material across neutral Sweden.
There were no realistic and viable plans drawn up by the General Staff for an amphibious invasion of the UK. Sea Lion was a hypothetical notion, a study into a possibility, and never really amounted to anything more than a discussion. It was quietly dropped in 1940 and never picked back up, partly because Hitler never really believed in conquest and occupation of the UK in any case, instead believing that the 'English', as an eminently sensible people, would undoubtedly negotiate some sort of terms for cessation of hostilities before it ever came to that.
The Battle of Britain is a great victory in terms of the morale boost it provided, but not much more than that in reality. Had the RAF lost, then Luftwaffe freedom to operate over the UK would have undoubtedly led to more widespread destruction and loss of life, and our supply chain would possibly have become even more strained than it did in the period after 1940, but it wasn't some sort of decisive moment that prevented German invasion, as that simply was not on the cards at any point, no matter what the public was being told at the time.