The German plan was to use its field army in Cracow's defense rather than a garrison. Two separate forces were used; one moving southwest of Cracow along the Carpathian hills, struck directly at Ivanov's left; the other, operating from Hungary, threatened the Russian rear. These two divisions struck at the same time and the Russians found it necessary to fight rear actions as they moved forward. They were doing this with reasonable success and working their way toward Cracow, when, on the 12th of December, the Austrian forces working from Hungary carried the Dukla Pass. This meant that the Austrians would be able to pour troops down into the rear of the Russian advance, and the Russian army would be cut off. Dmitrieff, therefore, fell rapidly back, until the opening of the Dukla Pass was in front of his line, and the Russian army was once more safe. Meanwhile the renewed siege of Przemysl was going on with great vigor, and attracting the general attention of the Allied world. The Austrians attempted to follow up their successes at the Dukla Pass by attempting to seize the Lupkow Pass, and the Uzzok Pass, still further to the east, but the Russians were tired of retreating. New troops had arrived, and about the 20th of December a new advance was begun. With the right of the army swinging up along the river Nida, northeast of Cracow, the Russian left attacked the Dukla Pass in great force, driving Austrians back and capturing over ten thousand men. On Christmas Day all three great western passes were in Russian hands. The Austrian fighting, during this period, was the best they had so far shown, the brunt of it being upon the Hungarian troops, who, at this time, were saving Germany. Meantime von Hindenburg was pursuing his movement in the direction of Warsaw. The Russian generals found it difficult to obtain information. Each day came the chronicle of contests, some victories, some defeats, and it soon appeared that a strong force was crushing in the Russian outposts from the direction of Thorn and moving toward Warsaw. Ruzsky found himself faced by a superior German force, and was compelled to retreat. The Russian aim was to fall back behind the river Bzura, which lies between the Thorn and Warsaw. Bzura is a strong line of defense, with many fords but no bridges. The Russian right wing passed by the city of Lowicz, moved southwest to Strykov and then on past Lodz. West of Lowicz is a great belt of marshes impossible for the movement of armies.