That's a good question. I think this system needs revision. The idea of the counter-attack was the attrition you experience from having two units scuffle. Generally speaking I would say it would have to use up the action, and it should be the attacked party's choice whether they use the counter-attack. I can see this leading to some weird situations either way, however. If a unit is attacked by 2 separate enemy units in a round, would they really inflict no casualties on the second if they use their counter-attack on the first? Unlikely.
EDIT: Reading this again, the way I wrote is so that you can only counter if the unit is destroyed by the incoming attack. This means the counter is the final action of this unit before it is removed from the board, so it does not matter whether this exhausts the unit or not.










