When they learnt that Cæsar was approaching the Meuse and the Rhine, the Usipetes and Tencteri had concentrated their forces towards the confluence of those two rivers, in the most remote part of the country of the Menapii, and had established themselves on the river Niers, in the plains of Goch. Cæsar, on his side, after leaving Venloo, had borne to the right to march to the encounter of the enemy. Since, to the north of the Roer, there exists, between the Rhine and the Meuse, no other water-course but the Niers, he was evidently obliged to advance to that river to find water: he was four miles from it when he met, at Straelen, the German deputation. The vanguard, consisting of 5,000 cavalry, marched without distrust, reckoning on the truce which had been concluded. Suddenly, 800 horsemen (all at the disposal of the Germans, since the greater part of their cavalry had passed the Meuse) appeared bearing down upon Cæsar’s cavalry from the greatest distance at which they could be seen. In an instant the ranks of the latter are thrown into disorder. They have succeeded in forming again, when the German horsemen, according to their custom, spring to the ground, stab the horses in the bellies, and overthrow their riders, who fly in terror till they come in sight of the legions. Seventy-four of the cavalry perished, among whom was the Aquitanian Piso, a man of high birth and great courage, whose grandfather had wielded the sovereign power in his country, and had obtained from the Senate the title of “Friend.” His brother, in the attempt to save him, shared his fate. This attack was a flagrant violation of the truce, and Cæsar resolved to enter into no further negotiation with so faithless an enemy. Struck with the impression produced by this single combat on the fickle minds of the Gauls, he was unwilling to leave them time for reflection, but decided on delaying battle no longer; besides, it would have been folly to give the Germans leisure to wait the return of their cavalry. Next morning their chiefs came to the camp in great numbers, to offer their justification for the previous day’s attack in defiance of the convention, but their real object was to obtain by deception a prolongation of the truce. Cæsar, satisfied at seeing them deliver themselves into his power of their own accord, judged right to make use of reprisals, and ordered them to be arrested. The Roman army, then encamped on the Niers, was only eight miles distant from the Germans.[303] [Sidenote: Rout of the Usipetes and the Tencteri.]