kr79:
Yes the british forces did hold of the invasion in 1940 due to better aircraft radar and better tactics from fighter command and churchills oratory helping motivate people.
But without hitler dropping a clanger by attacking russia then the USA getting involved theres no way the British could have held the Germans off in the long term. And certainly not mount an invasion back to the mainland.
Churchill knew all that regardless of what happened on the Eastern Front.However the fact is without the Spitfire and continuing development in naval and aircraft technology and manufacturing the Battle of Britain would have been lost and Britain would have been invaded and therefore US couldn’t have got involved.While the combined loss of the battle of the atlantic and/or the failure of the D Day invasion of Europe would have had the same effect assuming that naval and air superiority hadn’t been maintained throughout the war and that superiority in large part depended on British engineering and design.