In a showdown, when do When I Attack/When I Defend abilities trigger, when can players play action cards, and what is the sequence before damage is assigned?

Ruling: During a showdown, When I Attack/When I Defend abilities trigger first on an initial chain where action cards cannot be played. After the initial chain resolves, the attacker gains focus and can play action cards to start new chains, then focus passes to the defender who can play action cards, and this continues until both players pass, after which damage is assigned.

Sequence:

• Initial chain forms with When I Attack/When I Defend triggers and any reactions (no action cards allowed)
• Initial chain resolves backwards
• Attacker gains focus and can play action cards to start new chains
• After attacker's chain resolves, defender gains focus and can play action cards
• Focus passes back and forth between attacker and defender
• Once both players pass, damage is assigned

Nuances:

• Action cards like Stand United or Falling Comet cannot be played during the initial chain with triggered abilities
• Only after the initial chain resolves can the attacker use action cards