Germany would have been beaten in roughly the same time without the USA entering the European war. The USSR was too strong. Could Britain have beaten Germany without the USA and the USSR? Yes, due to the Commonwealth backup, but in much longer time. Probably the common influence of a war without end and a dismally failed invasion would have led to Germans becoming tired of the war and the regime breaking down economically.

An invasion might have been successful very shortly after Dunkirk, but there was no preparation of an invasion at that time because the quickness of winning the war in France surprised also the Germans.

Wolfgang