Tacitus ("De origine et situ Germanorum" XXIX) described the Batavi as the bravest of the tribes of the area, hardened in the Germanic wars, with cohorts under their own commanders transferred to Britannia. They retained the honour of the ancient association with the Romans, not required to pay tribute or taxes and used by the Romans only for war: "They furnished to the Empire nothing but men and arms", Tacitus remarked. Well regarded for their skills in horsemanship and swimming—for men and horses could cross the Rhine without losing formation, according to Tacitus. Dio Cassius describes this surprise tactic employed by Aulus Plautius against the "barbarians"—the British Celts— at the battle of the River Medway, 43: