@LuckyDay:
of course, if you look at how Russia got a 6 month headstart getting into the war and it is 6000 km from washington dc to berlin as compared to 1400 km from moscow to berlin, and still russia only made it to berlin first by a matter of days…
Well, in fact, USSR (not Russia) could had arrived a couple of months before, maybe even more. But Stalin wanted ensure soviet influence sphere on Balkans and eastern Europe, so he stopped assault on Poland and only made a full advance to west once they ensured Romania, Bulgaria and Yugoslavia. If USSR had chosen a eary push against Berlin, they could take whole Germany for them, but Balkans would be lost to the british. Of course, Stalin waited to see how D-Day worked, if failure, you can guess he would rush to Berlin and later Paris
War in Europe was won before D-Day (soviets would win anyway alone at that date), that’s credit for soviets. But D-Day saved France from stalinism, that’s credit for USA & UK
By the way, I think Switzerland should counted as impassable (similar to Himalayas and Sahara). Mainly for the reasons stated later