Meanwhile the Russians were following up their victory with a vigorous pursuit. Von Hindenburg’s army divided into two, one portion retreating through Tapiau to Königsberg, the other pressing in a south-westerly direction towards Allenstein, and the fortresses of Thorn and Graudenz. The former portion safely reached its destination, which was invested by the Russians on the 25th. On the same day Tilsit was formally occupied. Meanwhile the main Russian army, meeting with practically no resistance, pushed on along the line of the railway, occupying Angerberg and Korschen. By this time, however, heavy German reinforcements had come up, and the advance began to be contested with increasing determination. For three days there was vigorous fighting in the neighbourhood of Allenstein. Then, after inflicting heavy losses on the Germans, Rennenkampf entered the town and again forced von Hindenburg to retreat. The action, however, was not a decisive battle comparable with Gumbinnen, and the Russian advance became slow. Further fierce fighting, most of which resulted satisfactorily to the Russians, took place further to the south around Soldau and Nesdenberg. The Russians, after a remarkably quick advance through very difficult country, had now come within hail of the line of the Vistula. The line was protected by three first-class fortresses covered by at least four army corps in addition to the armies which had been driven back by the Russians. It was the critical moment of the campaign. With their heavy numerical superiority and strongly fortified position, the Germans would be sure to make a more determined resistance, and in greater force than any which the Russians had yet had to meet. In attacking the line Rennenkampf would be handicapped by a lack of heavy siege artillery, and by the numerical inferiority of his forces. On the other hand his troops comprised some of the finest fighting material in the world, they were flushed with victory and could be relied upon to make a tremendous effort to win the greatest triumph of all. If they could drive the Germans over the Vistula and bring up sufficiently large forces to invest Thorn, Graudenz and Danzig, the northern route to Berlin would be open to them as far as the Oder. The beginning of the end would indeed have arrived. The Russians accordingly pushed forward. But they did not advance much further on the road to the Vistula. An immense German army, heavily supported by artillery, including numbers of the heavy siege guns which had already proved themselves to be the Kaiser’s most potent weapons, awaited the Russians in a strong position in the neighbourhood of Osterode, midway between Allenstein and Graudenz.