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Combat tactics are determined for both sides at the beginning of a battle. Tactics last for 24-hour periods after which they are subject to change at the beginning of every 24 hour period the battle still goes on.
Each side has a weighted list of available tactics to choose from based on the current battle phase, commander skill (and reconnaissance advantage), researched techs and the fulfillment of other special prerequisites (as shown below). If successfully pulled through tactics may provide damage, defense, movement speed, and combat width modifiers. If an enemy's tactic is countered, they do not receive the benefits from their tactic choice.
Battle reports in progress show the combat tactics being used by each side for the current day, with a big red 'X' on a tactic if it is being countered. In the middle there is a button that displays a list of all available tactics (with unavailable tactics padlocked).
Note: Reconnaissance advantage is determined by comparing the sum of recon values in all divisions in combat of each side. It translates to a flat bonus equal to 5 leader skill points. The side with higher total leader skill points has combat "initiative" to pick a counter. The side with initiative will pick their tactic after the other side, and if a tactic is available to counter the other side's tactic its weight will be multiplied by 1 + 0.35 * difference in skill points. Reconnaissance advantage doesn't help dodge counters, that is, if the other side's tactic can't be countered and a certain available tactic is countered by the other side, this certain tactic has the same chance of being picked.