What happens if a surprise defender is stunned and the attackers dont have enough might to conqueoer

The attackers get recalled to base and don't conquer — the stunned surprise defender survives, establishes control, and the defender conquers the battlefield.

A stunned unit contributes 0 might to combat damage (Rule 423.1.b), but still requires damage equal to its full might to be killed (Rule 423.1.c). Since the attackers' total might is below that, they deal non-lethal damage and the defender survives. During Combat Cleanup, because a defending unit is still present, all attacking units are recalled to their base (Rule 466.1.a.2) — this Recall is not a Move and triggers nothing (Rule 454/451.1).

With the attacker gone, the defender is the only player with units remaining, so they establish Control and Conquer, scoring immediately even though it's the attacker's turn — stun doesn't prevent establishing control or conquering (FAQ 8498, 10965). This only fails if the defender had already scored that battlefield this turn.