It was a greater feat of Roman policy than that which enabled it to become master of Gaul, that it knew how to retain the mastery, and that Vercingetorix found no successor, although, as we see, there were not entirely wanting men who would gladly have walked in the same path. This result was attained by a shrewd combination of terrifying and of winning--we may add, of sharing. The strength and the proximity of the Rhine army was beyond question the first and the most effective means of preserving the Gauls in the fear of their master. If this army was maintained throughout the century at the same level, as will be set forth in the following section, it was so probably quite as much on account of their own subjects, as on account of neighbours who afterwards were by no means specially formidable. That even the temporary withdrawal of these troops imperilled the continuance of the Roman rule, not because the Germans might then cross the Rhine, but because the Gauls might renounce allegiance to the Romans, is shown by the rising after Nero's death, in spite of all its weakness; after the troops had marched off to Italy to make their general emperor, an independent Gallic empire was proclaimed in Treves, and those soldiers who were left were made bound to allegiance towards it. But although this foreign rule, like every such rule, rested primarily and mainly on superior power--on the ascendancy of compact and trained troops over the multitude--it by no means rested on this exclusively. The art of partition was here successfully applied. Gaul did not belong to the Celts alone; not merely were the Iberians strongly represented in the south, but Germanic tribes were settled in considerable numbers on the Rhine, and were of importance still more by their conspicuous aptitude for war, than by their number. Skilfully the government knew how to foster and to turn to useful account the antagonism between the Celts and the Germans on the left of the Rhine. But the policy of amalgamation and of reconciliation operated still more powerfully. [Sidenote: Policy of amalgamation.]