Hindsight is 20/20

Hindsight is 20/20. Really? That is one of the stupidest things I've ever heard. If hindsight is 20/20, and I am obviously not able to see it myself, can someone please tell me what would have happened if Hitler conquered England in 1941, instead of attacking Russia? Since MY hindsight is NOT 20/20, I'd like someone to tell me exactly how hindsight it 20/20. Thank you.