If a unit is moved to a battlefield during an ongoing showdown (after 'When I Attack' and 'When I Defend' triggers have already resolved), does that unit's 'When I Attack' trigger activate?

Ruling: Yes, 'When I Attack' and 'When I Defend' triggers activate anytime a unit joins a combat, including after combat has already started. The unit must be under the attacking player's control to be considered an attacker and trigger 'When I Attack' abilities.

Sequence:

• Showdown begins and initial 'When I Attack' and 'When I Defend' triggers resolve
• If a unit is moved to the battlefield during the showdown and is under the attacking player's control, it becomes an attacker
• That unit's 'When I Attack' trigger goes on the chain
• If the same unit is retreated and then moved back in again, the trigger activates a second time

Nuances:

• The unit must be under the attacking player's control to trigger 'When I Attack' abilities; if it's the opponent's unit, it does not trigger
• This rule is explicitly stated in the FAQ rather than the Core Rules