If I enter an empty battlefield and my opponent moves in a unit as a response, who is the attacker and who is the defender?

Ruling: When you enter an empty battlefield and your opponent responds by moving in a unit, the first showdown must fully resolve first (ending when both players pass with focus on an empty chain). Then a second combat showdown begins with you as the attacker, since you were the one who applied contested status to the battlefield. This is known as a "surprise defence."

Sequence:

• First showdown begins and must fully resolve (both players pass with focus on empty chain)
• First showdown ends without anyone having control since both players have units at the battlefield
• Second combat showdown begins with the original player (who entered empty battlefield) as attacker
• Control is only established after the second showdown ends

Nuances:

• The opponent can score if they win the second combat showdown
• Even though the opponent has a unit there after their response resolves, the first showdown continues as normal since it already began