If an opponent moves to an occupied base, passes it, and the defender uses Rebuke, can the opponent use Challenge before Rebuke resolves?

Ruling: No, once Rebuke is on the chain, only reactions can be used. Actions can only be played after the chain resolves completely.

Sequence:

• Rebuke is played and goes on the chain
• Only reactions can be played while the chain exists
• The chain resolves completely
• Actions can be played again, but the unit targeted by Rebuke is already gone

Nuances:

• Even after the chain resolves, Challenge cannot target the unit because Rebuke's effect has already removed it