Jamieson, Fausset and Brown on Deuteronomy 19:8-9

8-9. And if the Lord thy God enlarge thy coast--Three additional sanctuaries were to be established in the event of their territory extending over the country from Hermon and Gilead to the Euphrates (see Ge 15:18; Ex 23:31). But it was obscurely hinted that this last provision would never be carried into effect, as the Israelites would not fulfil the conditions, namely, "that of keeping the commandments, to love the Lord, and walk ever in his ways." In point of fact, although that region was brought into subjection by David and Solomon, we do not find that cities of refuge were established; because those sovereigns only made the ancient inhabitants tributary, instead of sending a colony of Israelites to possess it. The privilege of sanctuary cities, however, was given only for Israelites; and besides, that conquered territory did not remain long under the power of the Hebrew kings.