Was at war with red and my stack of 4 units was pillaging an administrative center. However I was forced into peace because I had done to well, and the next turn my army was just stuck there.

If I try to move the army or do anything with it, I get this notification. All I can do with them is pass their turn while they trespass. Am I overlooking something?

Trying clicking or dragging that army's movement towards a tile that's outside of that region, that's how it works for me when my armies are found trespassing directly after war... as the game only allows for movement in enemy territory if your army thinks that's the shortest way to get from Point A to Point B.