The problem with standalone battles, particularly against the AI, as I am guessing your's was, is that no consideration is given to force preservation and so casualties are often much higher than in real engagements, where one side or the other would give way when things went south.
If you ever get the chance to play against other people in a campaign setting, where keeping your troops alive is paramount, you will typically see a lot more caution, a much more palatable casualty rate, and less conclusive battles, as was the actual norm.