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:
In Unity, you have to click "Change Army Settings" (also under Armies):
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:
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