I think that after you declare war on someone a battle line type are of control should appear ala the cultural borders. and your lines would sort of be dictated by where your troops were and when defending any nearby troops would be considered to be under attack when an enemy tried to breach the battle line and enter your zone of control, certain units (special forces etc would be immune to this and could be used to carry out missions "behind enemy lines" as would aircraft). This would make warring way more realistic and fun I think.