Von Hindenburg now proved himself to be a leader of remarkable skill and resource and he performed as brilliant a feat of generalship as the war has yet produced. Only a year before he had taken part in the manœuvres in East Prussia, and was acquainted with every inch of the ground. It was even stated that he had already solved the exact military problem with which he was now faced, and in the same locality. In addition he enjoyed the advantage of outnumbering the Russians by at least two to one. These factors practically decided the battle. The district around Allenstein and Osterode is of the worst possible description for an invading force. It is a mass of lakes, swamps and forests, and an intimate knowledge of the locality is essential for the success of any military operations there. There are almost insuperable difficulties in the way of transport alone. Utilising his advantages to the full, von Hindenburg lured the Russians towards Tannenberg to the south-east of Osterode. The Russians, realising that a successful offensive was their only chance, blundered forward. They pressed on until they found themselves in a position where their flanks rested on more or less solid ground, but their centre was backed by a vast swamp. Then von Hindenburg struck his blow. An immense force was hurled against the Russian right. A desperate encounter followed, but sheer weight of numbers gave victory to the Germans. The Russians were forced back on to the swamps. A similar attack on the Russian left was equally successful. What followed was not a battle; it was one of the most hideous slaughters history has known. The Russians were unable to manœuvre on the swampy ground; the Germans, on the other hand, were in possession of the solid higher ground and free to move at will. From three sides they poured a murderous fire into the helpless Russians, forcing them deeper and deeper into the swamps. Guns sank in the mud, horses were unable to move, men stood up to their waists in the deadly slime. The carnage continued until nightfall, when Rennenkampf managed to escape with a remnant of his army, leaving Generals Samsonov, Martos and Pestitsch among the thousands of slain. Thus von Hindenburg won the battle of Osterode and obtained ample revenge for his defeat at Gumbinnen. The battle caused a complete reversal of the campaign. The route to Berlin via the north was not only barred, but the Russian advance was turned into a retreat. Hopelessly outnumbered, Rennenkampf was forced back on Allenstein. Every foot of the way was contested, but bit by bit he had to give up the results of his victorious move forward. Allenstein and Intersburg were in turn evacuated before the merciless pressure of the advancing Germans. The troops investing Königsberg were recalled. It was not until the frontiers were almost reached and strong reinforcements came up from Kovno and Grodno that the German advance was checked and finally held.