Good concept - but i suggest making the map run horizontally. Would be much better for keeping the eyes on - could have more of the battle field before you.
+The terrritory sizes should be of the same scale - pointing to the disparity between the trench territory sizes and the middle section territiory sizes.
+ For gameplay - maybe you don't need -1 for the middle territories, as people would just leave them there and they would all reduce to 1 or neutral (depending on how you set it up). Instead, you could have positions within the trenches, that border the middle section, as "firing position" (or somethink of the like) which could attack (or bombard) all the territiories on their respective half of the middle section.
{Though, having them all reduce to neutral would mean that people would probably take a side, build up their armies, and get ready for a charge - though this could just lead to drawn out stalemates, until someone cracks, which could be interesting in intself, but i think people would get over the gameplay setup pretty quick if this happened every time}
Good luck.