If Player A passes focus and Player B passes focus, does combat have to start or can Player A then choose to play an action?

Ruling: No, Player A cannot play a card after both players pass focus. Combat happens when both players voluntarily pass focus.

Sequence:

• When a unit is moved to the battlefield, "When I attack" then "When I defend" are added to the chain
• Player B has the opportunity to react (passing priority)
• Player A then has focus for a new chain, then Player B
• This cycle continues until both players pass focus
• Once both players pass focus, combat begins

Nuances:

• Focus (the opportunity to play Actions) is separate from priority
• Priority is passed starting with the player with Focus when a chain is created, then focus automatically passes
• When any chain item is placed, the player who placed it gets priority first