Can you play a spell in response to an opponent moving units to a battlefield you hold?

Ruling: Yes, when an opponent moves units to a battlefield you hold, it starts a showdown. If they pass focus, you can then play a spell like Foxfire.

Sequence:

• Opponent moves units to your battlefield, starting a showdown
• If opponent passes focus without playing anything, you can play a spell
• If opponent plays a spell first, after the chain resolves you can still play your spell (unless it was hidden, in which case you can react immediately)
• Combat only resolves when focus passes for each player in a row
• Everyone always has an opportunity to play an action after another player played an action

Nuances:

• If the opponent plays a hidden spell, you can react to it being played without waiting for the chain to resolve
• Each time a player plays a spell (unless you have a reaction), the chain needs to resolve first, then focus passes and the process can start again