Official rule text
39.2 A system tile is on the edge of the game board if any of its sides During the “Ground Combat” step of an invasion, if the active are not touching another system tile. player has ground forces on a planet that contains another player’s a The Ghosts of Creuss home system and the wormhole ground forces, those players resolve a ground combat on that planet. nexus are on the edge of the game board. To resolve a ground combat, players perform the following steps: RELATED TOPICS: System Tiles 42.1 STEP 1—ROLL DICE: Each player rolls one die for each ground
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Source: Twilight Imperium: Fourth Edition - Living Rules Reference: Prophecy of Kings
Expansion: Prophecy of Kings
Version: 2.0
Reference: Rule 39.2