323.4. 3a. All Units that have Lethal Damage marked on them are killed and placed in their owners' Trash. See rule 142.4. For more information on lethal damage.
323.5. 4. Players lose control of any controlled Battlefields without their Units occupying them if the turn is in an Open State and there is no Showdown or Combat ongoing there.
323.8. 7. Mark a Combat as Staged at each Battlefield that Contested was applied to that have Units present controlled by opposing players, and that has no ongoing Combat.
323.9. 7a. If Units of two opposing players are no longer present at a Battlefield that has a Combat Staged before it has opened, the Combat will cease being Staged
323.11. 9. If the current state is a Neutral Open State and one or more Showdowns are Staged at Battlefields without a Combat staged, the Turn Player chooses one of those Battlefields. A Showdown begins there.
323.12. 10. If the current state is a Neutral Open State and one or more Combats are Staged at Battlefields, the Turn Player chooses one of those Battlefields. Combat begins there.
323.13. 10a. If the current state is Showdown Open State and Combat is Staged at a Battlefield where there is a Non-Combat Showdown ongoing, that Showdown becomes a Combat Showdown.