Empires of the Word: A Language History of the World. Nicholas Ostler

distinct dialect areas. But even when, as in the golden ages of the Han and the Tang, the barbarian threat was effecively countered for centuries at a time, the consciousness of barbarians at the gate still remained, naturally causing a greater sense of unity in the population. The external threat of invasion kept the Chinese focused on what they had to lose; and recurrent partial failures of the centre’s defences against it kept the north of China in flux, and so perversely maintained the cohesion of its spoken language.