Well they player playing the nation just doesn't do anything on its secluded (no connections with other provinces) and his invasion army is just there.. waiting.. till the dungeon master decides it time to throw in that army and you teleport that army to where you want (which could be a province where there is his nation specific site
)
The best thing for this however (since I agree your idea is nice too) is that it will almost certainly work.. whereas a hidden commander might just attack... or be discovered.. or NOT do what it's supposed to do after it changes to what it should be.
If it autosummoned units that would work for sure.. BUT that couldn't be commanders..
I agree that for SP my idea isn't usefull and since most play is SP we need to see how we can make your idea stick best.
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fantasy and
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Gang Wars.