Well, the error notice when you combine the two armies is in fact correct, and the "emptiness" of the one army is not related to that.
The empty army itself is a bug. Do you remember how you did that? I will look at the code to see how this can happen. But anything that you remember might help me to track it down.
OK, here is what is going on with the concentration. You apparently moved one African Infantry unit under the command of Hanno. You moved that unit into Hannibal's army (possibly with Hanno) and then you recalled Hanno to Carthage.
So now Hannibal has that one unit in his army which is not eligible to move with him. It ws already moved by Hanno. Then you activated Hannibal, making him the current leader. You cannot combine armies or concentrate units into an army when the destination army has a unit that isn't eligible to move with the current leader.
In this case the rule of "a unit may only be moved by one leader in a turn" is admittedly creating a superficially strange technicality. But that is the cause, not the empty army.
This technicality has a purpose, however. You cannot "cascade" units in movement from one leader to another. Otherwise a unit could move all around the map under a succession of leaders.
When you transfer units from one leader's army to another leader's army, you are pretty much guaranteeing that the second leader isn't going to be able to do anything that turn, even if he's activated.
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