Well Lord Halifax , the man who Churchill supported to be prime minister after Chamberlain was forced out , proclaimed Hitler to be more of a visionary than a gangster. Sympathy for fascism was rife in the British establishment. Even Churchill , who loathed the Nazis was soft on Mussolini.
For what ever reason ,the Brits squirmed and wormed about in the 30s and in the run up to Munich. They effectively blocked attempts by Stalin to enactthe surviving alliences from WW! and later after the USSR joined the League of Nations to enact the doctrine of collective security against the emerging Nazi threat.
Unfortunately the history of ww2 has got overwritten by the History of the Cold War.
I think any US president would have had difficulty great difficulty in bringing the US into the war. Possibly even FDR would not have managed without Pearl Harbour and Hitler declaring war on the US..
However long before then , Americans had fought with great distinction in Spain - The Abraham Lincoln brigades of volunteers and the Flying Tigers in China .
Yes Us aid was important for Stalin , the Soviets built all their tanks but had no industry to build light trucks and jeeps which were essential for the mobility of the Soviet army. Without it , Stalin would still have defeated Hitler but it would have taken a lot longer and been bloodier. And at a loss of over 20,000,000 dead he probably would not have worried too much about losing a few million more..
As regards the D day and all that there were 7 German divisions in France as oppose to 175 in the East.
AJP Taylor on the origins of the second world war is still worth reading