It might have been attempted before, but let us look at this little ‘what if’
Germany is successful in Barbarossa. Leningrad falls in August 1941. Moscow goes in October.
Germany consolidates and Russian counter offensives do no not make any inroads.
There are two directions this can go:
Ukraine in 1941: limited offensives to secure Donbass
Case Blue in 1942 is successful and the Caucasus oil goes to Germany.
We need to also take into account that Brooke was off to Russia to evaluate f they could hold on. So there was serious doubts around.
With Russia out of the war (or at least severely maimed), Germany can turn to UK/US.
1942:
Bolero. Building up of forces in the UK, But how does US react to Russia out of the war? Will they focus on Pacific? Dropping UK?
UK losing Singapore and Burma
North Africa can be seen as a drain on resources which must be preserved for the invasion
Sea Lion in 1942 will meet limited US forces.
RN is still there
1943:
How much can Germany have consolidated their gains and bringing it into the German war economy?
US forces in UK not small anymore
US airpower is getting there (unless divided to Pacific)
Will US give up on UK?
If Tube Alloy is going to be pursued with a vengeance, UK might be able to go it alone if need be, but it is a long shot in 1942/43 to be assured it will work.
If UK is pulling out of Africa, the Suez canal goes. Does India go as well then?
Will Germany be severely weakened after a successful Barbarossa?
Ivan