Prior to the time of Valen, the Minbari's greatest spiritual leader,
there was killing of Minbari by Minbari. There were three warring castes,
which he pulled together when he formed the Grey Council (three from each
caste forming nine, Worker, Warrior, Religious Caste). Since that time,
and the Minbari have been in space for well over a thousand years, they
have pretty much hewn to that rule; it is their greatest taboo. Once the
three sides were integrated, the warrior caste mainly contented itself
with external threats.

jms