340. A Showdown begins when Control of a Battlefield is Contested and the turn is in a Neutral Open State.
340.1. If Control of a Battlefield is Contested between two players, then a Showdown will be opened as the first step of Combat. See rule 437. Combat for more information.
340.2. 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.