WWII was a failure of Versailles, not of isolationism. And FDR's actions on behalf of China that provoked Japanese attack were hardly isolationist. Nor should we forget that isolationism, in the imperfect manner that it was followed, gave us the tremendous advantage of being the last power in the war, an advantage totally ignored by today's Washington tough guys who seem to have a policy that we are to enter any life and death contest bloodied and bankrupt.
When it comes to WWII, we had a lot of catching up to do. It was the war that got us out of depression after over a decade; we were ultimately not as prepared as we ought to have been. Not an advantage of isolationism, as evidently the aggressors came looking for us.