@Krieghund:
Foreign air units, friendly or otherwise, may not fly over the territories or land on the carriers of a power that is neutral (not at war with anyone). A neutral power’s air units may not fly over territories or land on carriers that do not belong to it.
Wow. Thanks for asking the question, Jmite, I hadn’t thought about this situation yet.
In summary, the only powers that start the game neutral are USSR and USA. So these powers can not fly over any non-USSR or USA territories until at war with an Axis power, and Allies cannot fly over USSR or USA territories until USSR or USA is at war with an Axis power.