No, they can’t, unless they’re already at war. Powers not at war may not move units into territories or onto ships belonging to other powers.
or into neutrals.
A couple more questions: When the US is not at war I understand it can’t move its ships towards Japan or Germany, BUT
1. Can it fly its Bomber over to London prior to being at war
2. Can it fly its fighter from Phillipines down to Australia
3. Can it retreat its boats from the SZ surrounding the Phillipines back towards Hawaii, India, or Australia
4. Can it land its aircraft on any of its allies territories
No to 1,2,4. Look for a “powers not at war” box in the rules.
For 3, you can move to any sea zone which is not adjacent to Japanese territory. So you can totally move toward Japan, eg to SZ16 or SZ17. Just remember that if you move to SZ54, you can’t use the naval base there until you are at war with someone.