In the settings, you will have to enable the "One Army Must Stand Guard" setting.
In the Flash client, it's the second checkbox under Armies:
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In Unity, you have to click "Change Army Settings" (also under Armies):
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Then scroll down a bit, past the move order and before the base army settings. You will find 3 checkboxes buried in that region,
one of which is the One Army Must Stand Guard setting:
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To have 1 army stay in its territory of origin, make sure that box is
checked. To have settings the way they are now, leave it unchecked.
Hope that helps!
Edited 4/24/2018 17:39:51