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

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