Plutarch Caesar 19

[1] His second war, directly in defence of the Gauls, was against the Germans, ¹ although previously, in Rome, he had made their king Ariovistus an ally. ² But they were intolerable neighbours of Caesar’s subjects, and if an opportunity presented itself it was thought that they would not remain quietly in their present homes, but would encroach upon and occupy Gaul.

[2] Seeing that his officers were inclined to be afraid, and particularly all the young men of high rank who had come out intending to make the campaign with Caesar an opportunity for high living and money-making, he called them together ³ and bade them be off, since they were so unmanly and effeminate, and not force themselves to face danger; as for himself, he said he would take the tenth legion alone and march against the Barbarians; the enemy would be no better fighters than the Cimbri, and he himself was no worse a general than Marius.

[3] Upon this the tenth legion sent a deputation to him, expressing their gratitude, while the other legions reviled their own commanders, and all the army, now full of impetuous eagerness, followed Caesar on a march of many days, and at last encamped within two hundred furlongs of the enemy.
  Now, the very approach of Caesar somewhat shattered the purpose of Ariovistus. For he did not expect that the Romans would attack the Germans, whose onset he thought they could not withstand, and he was amazed at the boldness of Caesar; besides, he saw that his own army was disturbed.

[4] Still more, too, was the spirit of the Germans blunted by the prophecies of their holy women, who used to foretell the future by observing the eddies in the rivers and by finding signs in the whirlings and splashings of the waters, and now forbade joining battle before a new moon gave its light. When Caesar learned this, and saw that the Germans kept quiet, he decided that it was a good plan to engage them while they were out of heart, rather than to sit still and wait for their time.

[5] So, by attacking their entrenchments and the hills on which they were encamped, he irritated them and incited them to come down in anger and fight the issue out. They were signally routed, and Caesar pursued them a distance of four hundred furlongs, as far as the Rhine, and filled all the intervening plain with dead bodies and spoils. Ariovistus, with a few followers, succeeded in crossing the Rhine; his dead are said to have been eighty thousand in number.