The reason we could not be isolationist in 1941, is because of our expansion in The Pacific, that came with the end of the Spanish-American War (Hawaii and The Philippines), and the rising rivalry with Japan, we had to greatly expand the Navy presence in the Pacific. We relied heavily on The Royal Navy to protect the Atlantic, and if that fell into German hands, we would have been in big trouble.
Yes, but that train sailed in 1898.
For all the flaws that may have tainted prior U.S. expansion efforts in the 19th century, at least they were aimed at expanding the U.S. and ultimately converting its territories to new states.
That all changed with the Spanish-American War. It was the first time the U.S. invaded and occupied foreign lands without having any interest in making them part of America.