sometimes I don’t understand the battle logic

[Alucardvincent]

I was trying to defeat some rebel armies (see my other post) I defeated them I province “A” and retreat to province “B” I wanted to destroy them once for all in theory when they retreat the have zero or very low morale, but when we arrived my army was the one defeated!! My army retreats and the rebels in pursue to province “C” and this time my army was the one totally destroyed!! I was like “what the heck happened?”

 

[Borne2Run]

Your army probably had lower morale or your general had bad rolls (decisions).

 

[Lofman]

Even if victorious your army could be in a bad shape as well, if so it may be a good idea to be a bit more defensive. Note that defensive bonuses can be quite significant, if you find yourself attacking and things are going badly and the enemy having defensive bonuses, try to retreat before your army is too badly mauled (keep in mind that you can't retreat directly).

 

[Cheexsta]

In EUR, more often than not a retreating army will arrive in its target province in the month following the lost battle, meaning they have a little time to recover some of their morale. I get the feeling that their morale recovered enough to defeat you in the follow-up battle and your army simply wasn't able to do the same.

Something I've found that helps is setting the game to "pop-up and pause" whenever you win a battle: this allows you to pursue your enemy immediately, and if they retreat to a province with a terrain bonus then you'll be the one to get the bonus.