The lull which followed the battle of Ypres was not entirely due to the winter season or to the Flanders mud, for both sides had other reasons for quiescence in the West. The Germans had definitely failed in their original plan of destroying the French armies before the Russians could intervene, and they were now threatened with the ruin of their Austrian ally and the invasion of their own Silesian borders. The steam-roller, which had been moving to and fro across the Polish plains, seemed to have at last secured a solid impetus in the forward direction which might conceivably carry it to the Brandenburg Gate by Christmas. Württemburgers and Bavarians might afford to keep their eyes fixed on the Channel ports and their troops in Belgium; but the affections of Prussians were set on their homes in the East, and Hindenburg was calling for reinforcement more clamantly than the Western commanders. Defence was for many a month to be the German strategy in the West, and, in spite of the failure of their higher ambitions, they had secured a good deal worth defending. Belgium, with its great mining and other industrial resources, was theirs to relieve the strain on German labour and raw materials; from the Briey district in Lorraine they were drawing ores without which they could not long have continued the war; and the coalfields of northern France were divided between their owners and the invaders. The strain which the lack of these resources put upon the industries and shipping of Great Britain was incalculable, and the inability of the Entente to defend the French and Belgian frontiers or to expel the invader prolonged the war for at least a couple of years.