I suspected there was something to do with isolationists behind this. Some people berate the US for coming late to the war--and World War I.   But they don't seem to realize the strength of isolationists in the US. And the government has to pay attention to the views of the electorate. In 1916 Wilson won--narrowly--on the strength of "He kept us out of war". And in 1940 FDR's Republican opponent did not try to run on a similar platform--but there was strong sentiment in favor of keeping out. Even after Pearl Harbor it was not a given that the US would come in strongly against Nazi Germany--until Hitler solved the problem by declaring war on the US.