When units counterattack not the strength/number of their swords count but the strength of their shields, and also units who have special additional attacks like throwing weapons or first strike can't use them when counterattacking, therefore it's usually better to attack first than to be engaged in counterattack especially with those additional attacks. But not always, attacking Phantom Warriors first for example is worse than defending and letting them come, strangely, though if there's first strike or throwing weapons available then attacking first is still better as then they're killed without even harming the attacking unit. So I'm not entirely sure of the entire mechanism.
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