When can action cards be played during combat, and can both attacker and defender use them?

Ruling: Both attacker and defender can play action cards during a showdown, but only when they have focus and no chain is currently open. Actions cannot be played while any chain is resolving.

Sequence:

• When a unit moves to an occupied battlefield, a showdown begins
• Attack/defend triggers go on a chain first (only reactions can be played on this chain)
• After that chain resolves, the attacker gains focus and can play an action (defender can only play reactions on this chain)
• After that chain resolves, the defender gains focus and can play an action (attacker can only play reactions on this chain)
• This alternates back and forth until both players pass focus without playing anything

Nuances:

• Actions can be played in an open neutral state on your turn, or when you have focus during a showdown on either player's turn
• Reactions can be played any time you have priority
• Once any chain is open (from a card being played or ability activated), actions cannot be played until that chain fully resolves
• Focus stays with one player during a chain, but priority passes around
• Actions played during a showdown can target anything legal for that card, not just units in the current showdown