We know now (and have for some time) that in 1940, Germany did not have the capabilty of invading Britain in 1940. Operation SEALION, as envisaged by German planners, would have been a debacle for the Germans.
However, it has also been noted that observers didn't know that. It has been pointed out that in the previous year, Germany had gained victories that had seemed impossible. Thus by mid-1940, it seemed to many British leaders, many prominent Americans, and lots of other important figures though the German military was irresistible. Germany had demonstrated mastery of several new military techniques, including tanks and airborne troops. No one knew what other rabbits might come of their hat. Many observers thought Britain would have to make peace, or be destroyed.
OTL nothing much came out of the German hat at the time. Britain accepted the challenge of fighting, and the rest is OTL history. Germany did pull out some technical surprises later in the war, but too little and too late to gain any further victories, much less to win the war.
So what additional capability could Germany have had in 1940? Germany developed and deployed several highly effective new weapons and techniques during the war; that is, within 4 1/2 years of the time in question. Thus it is
prima facie plausible that Germany could have had any or all of these capabilities in 1940; I'm not talking about atomic bombs or lasers. Below are those I have thought of. I welcome additional suggestions.
- Hs 293 and Fritz-X glider bombs
- Type XXI U-boat
- Me 262 jet fighter
- Siebel ferry
- Dive bombing of ships at sea (the Germans figured out how to do it with Stukas by mid-1941)
Does Anyone Else Have Something Further To Add?
(Actually, I'll mention one thing that Germany never had, but could easily have acquired in 1940 - knowledge that their Enigma cipher system was compromised, and how it was done. This information was in the hands of French intelligence, and could have leaked to the Germans. A venal or careless French spook, for instance; or French failure to sanitize completely their codebreaking site before it was overrun.)
That brings us to the important question:
With some of or all of these capabilities in 1940, could Germany have executed an invasion of Britain?