This would by default be present in free flight, and maybe low intensity stock missions.
This would also be for the mission editor.
So, what I'm thinking is the enemy faction may not attack immediately, but will not allow things like bombers near their units and would send escorts to aircraft that get within a certain radius,and will only attack if fired at.
They would still allow aircraft to land at their bases, but would keep SPAAGs and such targeting them just in case, unless it was a logistics aircraft.
Ground units would not attack each other unless they see a SAM shoot down a friendly aircraft in self defense, which would cause them to fight for a minuite or so before stopping (lorewise this would be considered them being ordered to cease fire after being told it was an accident)
If you anger the other faction enough (eg. Nuking a base of theirs/sinking destroyer) then they will become all hostile like normal.