That "withdraw button" is a "rout button" really, game engine-wise you are forcing the selected unit to panic off the battlefield...
That's what I meant, sorry if I wasn't clear.
Well... you have the right to think that, but it is pretty common military tactics for a force to sometimes engage with the intention of withdrawing before the battle is "over" (whatever “over” means here). For instance, one might want to weaken the enemy, or delay him, or draw him out, or provoke him, or simply probe his forces
Sorry for being blunt: trying to apply that to a game engine one knows that it doesn't (can't) support it and then complain about it is what eventually led me to the 'dumb' response. Guerrilla tactics simply are not supported by the game, never have, never will. It has got 'Total' in the name = if you engage in a battle be prepared to be decimated, routed, annihilated. Expecting to be able walk away when it doesn't go well for you is a 'common' desire but utterly unrealistic in this game. You either win a battle, get annihilated or rout. And leaving the battle field during the battle is routing where the game engine is concerned - regardless what the reason is.You want to weaken the enemy? Then you annihilate his armies and\or take his settlements.
You want to provoke him? Then raid his settlements (raze and leave to revolt).
You want to probe his forces? Send a spy, or sacrifice a cheap unit to reconnoiter.
If you wish to use more realistic tactics then I can recommend the 'Order of Battle' game series (I totally enjoyed the 'Morning Sun' DLC) - Total War is the wrong choice for it.
Once more - sorry for being blunt. But this is not an advanced combat\strategy simulator - ignoring the limitations and expecting otherwise is simply willful ignorance.
Bookmarks