Apart from either switching your weapon(s) "off" by clicking on it/them so it goes blue instad of orange , or setting your range so the unit won't engage I don't know of one.
If there is another way I'd like to know!
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"Ki te mutunga" - "Through to the end".
There is no opportunity fire option in WinSPMBT or SPWW2 because it disadvantages the AI vs. a Human player. The AI has no way of deciding if this is or is not a good time to hold fire or let loose with everything it's got except in a very general way which are also the same "rules" that are applied to the opfire for the human player. It may be fun to hold fire and then let loose an ambush but the AI has no way of doing the same in return. The best you can do is adjust your the ranges your weapons open fire which is also a "cheat" which the AI cannot do in return but it's not as severe and requires more skill on the human players part to really make effective
Don
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If you find you are constantly reacting to your enemy's tactics instead forcing the enemy to react to yours, you are losing the battle..