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Question: Stuck in invasion mode

I am doing a scenario where there is a river. Invaders have come in on the other side of the river from where I have built my city thus far. They can't get to me, and I can't get to them. This would be fine, except that they won't give up and go away and all trade is suspended while the invading army is on the map. I can't build any more military buildings or bridges while in invasion mode. What to do?