Had the UK been captured or had it surrendered in 1940, it is plausible to imagine the European conflict ending at that point. With the US not involved at that point, without the UK as a staging post, and allowing for the technology available at the time, I'm not convinced the retaking of Europe would have happened.

Happily, all just pure speculation.

PS if you like military history, Anthony Beevors 'Stalingrad' is a truly great book - and details a truly terrible battle that was pivotal in the passage of WWII