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Special Gameplay: Trench Warfare
With trench warfare you can only assault from regions held continuously since the start of your turn. If you conquer a region (or re-conquer a region) you cannot assault further from that region during the same turn. The one exception being 'killer neutrals' (explained on the Gameplay Notes page) from which you may continue assaulting.
The game no longer involves steamrolling across the board. Instead you gradually advance your front!