The biggest isolationists were the Communists, after the Non-Aggression pact between Hitler and Stalin. They said the US should stay out of the “European Imperialists War”.
Yep. I was going to post that.
And when Germany invaded them, they turned on a dime so quickly that it made heads spin.
That was one of the key issues that caused Whittaker Chambers to fully break with the Communist Party, the Non-Aggression Pact.