Official rule text
42.2 STEP 2—ASSIGN HITS: Each player in the combat must 3. Status Phase choose one of their own ground forces on the planet to be 4. Agenda Phase destroyed for each hit result their opponent produced.
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Source: Twilight Imperium: Fourth Edition - Living Rules Reference: Prophecy of Kings
Expansion: Prophecy of Kings
Version: 2.0
Reference: Rule 42.2