What about the thème : the quest for identity in the novel?

The novel is a close examination of identity and the fear of the other, the foreigner, the outsider. More specifically it is a parable about the creation of an enemy that comes from that fear. The barbarian is a derogatory term that identifies a group as outsiders and vilifies them. The term is deployed effectively by the Empire to designate the nomads as different—lower, savage—and thus to define them as the enemy and justify their persecution. The hatred that grows from fear justifies the violence that perpetuates the fear—specifically of the other.