Timing of a Battle between Armies

[numero42]

a battle between two or more armies, ends only when one of the two sides escapes or is destroyed? is there no possibility of sending reinforcements? Because if that were the case, sieges would not exist. or at least they would be the shortest sieges in history. It would not even make sense to send armies to battle around. I send all the troops to the opponent's capital and if I have more troops I have almost certainly won without the enemy being able to react by calling troops to help.