In a showdown when attacking a Battlefield, who has priority to play the first card - the attacker or defender?

Ruling: The attacker (active player) has priority to play the first card in a showdown, unless there is an initial chain created by triggered abilities.

Sequence:

• If there are no "when I attack" or "when I defend" triggers, there is no initial chain and the attacking player has focus and can play the first action or reaction
• If there is a "when I attack" trigger only, the attacker creates the chain and remains the active player with priority
• If there is a "when I defend" trigger only (no "when I attack"), the defender becomes the active player and has priority first
• If both "when I attack" and "when I defend" triggers exist simultaneously, they are added to the chain starting with the turn player and proceeding in turn order, though it's unclear if there's an opportunity to react between them

Nuances:

• The active player in a chain is whoever created the chain
• When multiple triggered abilities trigger simultaneously, the turn player orders their triggers on the chain first, followed by other players in turn order
• If there is an initial chain, once it resolves the attacker loses focus without getting an opportunity to play an action