If your outpost is captured and you do not have any other captured territory on the board, you cannot march/attack a territory. You have to either wait to get respawned back to your territory or wait unit one of your allies reclaims your outpost for you.
Waiting time for the player be respawned back to his territoryRemember, if your original outpost is captured, but you still have captured territories on the board; you can still march or attack enemy territories from your captured territories.
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Empires & Allies Game GUIDE