If Control of a Battlefield is Contested between two players, then a Showdown will be opened as the first step of Combat. If a Showdown is already ongoing at that Battlefield, it will become a Combat Showdown and a Combat will initiate there. See rule 459. Combat for more information.
§ 344.1 Core Rules PDF (officieel)
§ 344 A Showdown begins when Control of a Battlefield is Contested during a Cleanup and the turn is in a Neutral Open State.
