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440. Combat can only occur between Units controlled by exactly two players.

440.1. In Modes of Play with more than two players, Battlefields with Staged Combats or Combats in Progress are Invalid Destinations for Moves of all kinds ( Standard Moves or otherwise) by Units controlled by Players who don't already have Units at that Battlefield. See_rule_REF_425.2.a_DOT for more information on Invalid Destinations.
440.2. In Modes of Play with more than two players, Battlefields with Staged Combats or Combats in Progress are Invalid to be chosen as a location to play one or more Units by a player not involved in that Combat by any means,
440.3. All choices that would result in a Combat occurring between more than two players simultaneously are invalid and ineligible to be completed.