If my opponent plays a unit with Ambush to a battlefield where they control a unit, can I react by killing their only unit to prevent the Ambush unit from entering play?

Ruling: No, you cannot react to prevent the Ambush unit from entering play. Units (including those with Ambush) immediately resolve when played and do not create a reaction window.

Sequence:

• Opponent declares playing the Ambush unit to a battlefield where they control a unit
• The Ambush condition is checked and validated at this moment
• The unit immediately finalizes and enters the battlefield
• The chain clears and Focus/Priority pass to the opponent
• No reaction window exists before the unit enters play

Nuances:

• Units and Gear do not linger on the chain like spells do
• You can only react to triggered abilities (like "When you play me" effects), not to the playing of the unit itself
• The Ambush location requirement is only checked when the unit is declared, not continuously