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.
The aggressors came looking for us bloodied and already involved with other enemies. Being last to fight was a huge advantage and one that our geographical separation should guarantee if we had prudent leadership.