Location on a frontier involves remoteness from the center of national, cultural, and political activities; these reach their greatest intensity in the core of the nation and exercise only an attenuated influence on the far-away borders, unless excellent means of communication keep up a circulation of men, commodities, and ideas between center and periphery. For the frontier, therefore, the centripetal force is weakened; the centrifugal is strengthened often by the attraction of some neighboring state or tribe, which has established bonds of marriage, trade, and friendly intercourse with the outlying community. Moreover, the mere infusion of foreign blood, customs, and ideas, especially a foreign religion, which is characteristic of a border zone, invades the national solidarity. Hence we find that a tendency to political defection constantly manifests itself along the periphery. A long reach weakens the arm of authority, especially where serious geographical barriers intervene; hence border uprisings are usually successful, at least for a time. When accomplished, they involve that shrinkage of the frontiers which we have found to be the unmistakable symptom of national decline. This defection shows itself most promptly in conquered border tribes of different blood, who lack the bond of ethnic affinity, and whose remoteness emboldens them to throw off the political yoke. The decay of the Roman Empire, after its last display of energy under Trajan, was registered in the revolt of its peripheral districts beyond the Euphrates, Danube, and Rhine, as also in the rapid Teutonization of eastern Gaul, which here prepared the way for the assertion of independence. The border satraps of the ancient Persian Empire were constantly revolting, as the history of Asia Minor shows. Aragon, Old Castile, and Portugal were the first kingdoms in the Iberian Peninsula to throw off Saracen dominion. Mountain ranges and weary stretches of desert roads enabled the rebellions in Chinese Turkestan and the border districts of Sungaria in 1863 to be maintained for several years.[387] [Sidenote: Centrifugal forces on the frontier.]