Their gloom was relieved by the halo which shone round Hindenburg's head. Rennenkampf was gone and all the faculties of the University of Königsberg conferred degrees on the victor to celebrate its escape. Reinforcements were sent to the frontier, and on 7 September Russia was invaded. The object of the offensive is not clear except on the assumption that Hindenburg's strategic acumen was defective, and that he thought he could turn the Russian right by an advance across the Niemen. But the difficulties were insuperable and the distances were vast. Even if he got to Kovno it would need far greater forces than he possessed to cover and control the illimitable land beyond; and between him and success lay swamps more extensive then the Masurian Lakes and the heavily fortified line of the Narew. He was, indeed, in his turn falling into Samsonov's error, and seems to have been saved from his fate mainly by the prematurely successful Russian defence. He was allowed to reach the Niemen at various points between Kovno and Grodno, but was unhappily prevented from committing his fortunes to the eastern bank by the Russian artillery, which repeatedly destroyed his pontoons as soon as they were constructed. Lower down on his right an attempt on the fortress of Ossowiec proved equally futile, because the Germans could find no ground within range solid enough to bear the weight of their artillery. The inevitable retreat began on the 27th, and it was sadly harassed by the pursuing Russians, especially in the forest of Augustowo, where Rennenkampf claimed to have inflicted losses amounting to 60,000 men in killed, prisoners, and wounded. By 1 October the Russian cavalry was again across the German frontier, and Hindenburg was called south to attempt in Poland to frustrate the Russian advance on Cracow which his turning movement in the north had failed to check.