The call was urgent, for the conquest of Galicia portended disaster to the Central Empires. Cracow was a key both to Berlin and Vienna; its possession would turn the Oder and open the door to Silesia, which was hardly less vital to Germany than Westphalia as a mining and manufacturing district. It would also give access to Vienna and facilitate the separation of Hungary, and all that that meant in the Balkans, from the Teutonic alliance. Even without the loss of Cracow, that of the rest of Galicia was serious enough; her oil-wells were the main sources of the German supply of petroleum, and her Slav population, once assured of the solidity of Russian success, would throw off its allegiance to the Hapsburgs and entice the Czecho-Slovaks on its borders to do the same. These prospects were not visionary in September 1914. Jaroslav fell on the 23rd and Przemysl was invested. Russian cavalry rode through the Carpathian passes into the Hungarian plain, and west of the San patrols penetrated within a hundred miles of Cracow. In her own interests as well as in those of her ally, Germany was compelled to throw more of her weight against the Russian front. The German and Austrian commands were unified under Hindenburg, and having failed on the north he now tried to stop the Russians by a blow at their centre in Poland. Here Ruszky was now in command, while Ivanoff with Brussilov and Dmitrieff as his two lieutenants controlled the armies in Galicia. Like every German general Hindenburg believed in the offensive being the best form of defence, and like all Germans in the advantage of waging war in the enemy's country. His plan of attack was a concentric advance on Warsaw along the three railway lines leading from Thorn, Kalisch, and Czenstochowa, combined with an effort to cross the Vistula at Josefow while the Austrians kept step in Galicia, relieved Przemysl, and recovered Lemberg. There was even a movement southwards from East Prussia which captured Mlawa, but it was only a raid which did not hamper the Grand Duke's contemplated counter-offensive. Warsaw had obvious attractions; Josefow was selected because it was far from Russia's railway lines but near to Ostrowiec, the terminus of a line which led from the German frontier; and the object of crossing the Vistula was to take in the rear the great fortress of Ivangorod lower down, and then to get behind Warsaw. The Grand Duke had divined these intentions, while he concealed his own by misleading the Germans into a belief that he proposed abandoning the Polish salient and retiring on Brest. His real plan was to stand on the east bank of the Vistula save for the defence of Warsaw which lies upon the west, and to counter-attack round the north of the German left wing under the guns of the great fortress of Novo Georgievsk. Rennenkampf was brought down to command this movement, while Ruszky took charge of the defence at Josefow.