I had an interesting thing happen to me last night. I have taken over Gaul and am currently allied with the Germans and neutral with the Britons. The Britons have not expanded beyond their starting provinces, though they were friendly with the Gauls and from the battle markers I can see they were in a long war with the Germans for a period of time. Recently, they marched a very large army led by a family member through German territory to Alessia (sp?) in NW Gaul. This town is my main German frontier town in that area and at the time it had a full garrison of 20 units, plus another half stack next to it. The Britons plopped their army down right next to this town. In the hopes of encouraging them to attack, I moved 3/4 of my forces away from this area towards the coast. Given that they were marchine across the territory of their former enemy and into mine with a large army and without permission, I assumed that they were there to attack. Instead, after I weakened my town they turned around and went home. I had another stack lurking a turn and a half south ready to pounce on them once they beseiged me, but I highly doubt the AI was smart enough to figure that out. Why did they retreat?