When does a Showdown begin?

A showdown begins when control of a battlefield becomes contested or when one or more units move to an empty battlefield.

According to Rule 340:

• 340.1: If control of a battlefield is contested between two players, a showdown is opened as the first step of combat.
• 340.2: If a battlefield is uncontrolled (empty) and becomes contested by a unit moving into it, a showdown is opened during the cleanup at the end of the action that caused the battlefield to become contested.

Important nuances regarding when a showdown does NOT begin:

• Moving a unit to a battlefield you already control does not trigger a showdown (it is simply a reinforcement).
• A showdown cannot start while a chain is currently resolving; it will stage and start at the first open state after the chain resolves.
• Only one showdown can exist at a time. If multiple showdowns are staged, they must resolve one after another.