Yes, they can do that even if the powers are at war. The presence of a sub belonging to an enemy power does not block movement.
Marsh
Is there any reason not to Attack russia G3 if JPN is going to Attack J3 ?
@barnee You could wait until G4, with a J3 & I3 DOW you get the +5 NO one more round. J3 happens after R3, so they wouldn’t get their NO for lend-lease that third round.
Not attacking G4 is probably unwise, unless you have been completely focusing elsewhere, Russia can DOW on R4 if they choose (and why wouldn’t they)
@mr_stucifer thanks for the reply.
I was thinking USA could Attack start of there turn either way. US East Coast Fleet could hit GIB
Edit
To Clarify
G doesn’t Attack G3 JPN does Attack J 3 , USA can then Attack any Axis Units USA 3. They don’t have to wait till turn 4
@barnee USA can declare war during the Collect Income Phase of their 3rd turn, which means they must wait until US4 to make any moves outside their normal restricted area.
@barnee Ah, I thought you were asking specifically about attacking Russia with japan on J3.
@mr_stucifer oh no my bad :) If JPN Attacks J 3 , I think G 3 should have attacked G 3 too.
Good question (sadly, I don’t have answers though, I just wanted to support you).