344. A Showdown begins when Control of a Battlefield is Contested and the turn is in a Neutral Open State.
344.1. If Control of a Battlefield is Contested between two players, then a Showdown will be opened as the first step of Combat. See rule 454. Combat for more information.
344.2. If Control of a Battlefield is Contested and the Battlefield in question is uncontrolled when it becomes Contested, a Showdown is opened during the Cleanup at the end of the action that caused the Battlefield to become Contested.