7518. Instead of putting up a united front against on common enemy, the medieval states frittered away their energy in internecine warfare.

✅ The correct answer is option A.
destructive : causing great and irreparable damage.

baneful : harmful or destructive.

pernicious : having a harmful effect, especially in a gradual or subtle way.

detrimental : tending to cause harm.

internecine : destructive to both sides in a conflict.

Synonym of internecine is destructive