On the east bank of the Vistula in Poland the Russians were mobilizing new armies which were shielded from attack by the river; once assembled they would cross the river to march west into Silesia. To counter the Russians' pending invasion of Silesia, Hindenburg advanced into Poland and occupied the west bank of the Vistula opposite from where Russian forces were mobilizing. He set up headquarters at Posen in West Prussia, accompanied by Ludendorff and Hoffmann. When the Russians attempted to cross the Vistula, the German forces under his command held firm, but the Russians were able to cross in the Austro-Hungarian sector to the south. Hindenburg retreated and destroyed all railways and bridges so that the Russians would be unable to advance beyond west of their railheads well short of the German frontier.