Just for clarification, the case that USA can’t land air in Russia would be based on quotes from the rulebook like this
"No air units can land in any territory that was not friendly
at the start of your turn, including any territory that was just
captured or converted from a friendly neutral by you this turn. "
and the assumption that USA declares prior to the Combat Move phase, which is after the start of the turn.
So which is it? Does Russian territory immediately become friendly when it becomes allied to the USA?
Have we been playing wrong if we are allowing USA to land stuff in Africa or London or Russia after they declare in this scenario?
If we interpret it this way, it would also be illegal for Russia to move units into China on the same turn they declare on Japan.
Without clear language in the rules one way or another, we’re going to need an official ruling on this one.