If I may... I'm really surprised by this answer. Please forgive my english as I don't practice it enough... but I want to make sure I understand right.
In Europe 1940's rule book, p.10, under Friendly Neutrals :
Friendly neutrals may not be attacked, and air units may not fly over them. They can be moved into (but not through) as a noncombat move by land units of a power that is at war
Say, a friendly neutral to me... I can't move during combat move, I can't move throught it (blitz) during non combat. But, from your answer, the ennemy can. So, actually, "sided" neutrals without troops is more open to his ennemy's than his friends.
From that rule, I thought all neutrals were stopping movement, regardless of sides. But seems I was wrong.
In clear, for instance :
There's no way Russia can rush through untouched Northwest Persia, but Axis could attack Caucasus from Iraq with armors through the same untouched Persia. Correct?
(By untouched, I mean no nation ever walked in, therefore no nation markers over it).