lol, I disagree. What would often happen is after the first few stages of the battle it would quickly become evident that if an army sat there and fired it would be wiped out, so they would create an ordered retreat. If the enemy pursued them they would lose the advantage of their superior position, so it was hard to have a decisive victory. A good general COULD get an enemy in a position where he could have a decisive victory though, but it was not common. Also, enemy armies would often surrender rather than being wiped out.
Not only would it be more realistic, but it would put more of an emphasis on precombat maneuvers and less on standing and watching.
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