Just curious how retreating works. Just had a disastrous series of events in a first-play through as Romans in First Punic War. Moved major Roman army of 60,000 or so along coast and had a battle with about 70,000 Carthaginians. The battle was a draw but since I was outnumbered and took heavier losses I decided to move back to where I had come from. Both sides had equivalent cavalry and were supplied and had almost the same values of commanders (Roman general was a three attack and the Carthaginian general had a four defense. Weather was fair and terrain was hills.
Lo and behold, despite trying to move away, each turn ended with a battle and the Romans stayed put. Three months and three battles later, and with the losses of about ten national morale points total, the Romans finally made it back whence they had come.
From what I know of ancient and medieval history, battles usually took place because both sides decided to fight. Otherwise, barring one side or the other having a huge disparity in cavalry, one side could just march away.