But the main reliance of Germany in the east was Austria, whose whole force, save for one or two corps borrowed by Germany to defend Alsace and four corps sent against Serbia, was available for the invasion of Russian Poland. If Austria could organize a resistance that would last for six weeks, Germany was prepared to do the rest. This she expected of Austria, and again her calculations were wrong.
A glance at the map serves to explain the opening moves in the eastern campaign. Russian Poland projects into Austro-German territory, and is nearly encircled by German East Prussia and Austrian Galicia. Russian mobilization had therefore to take place not at the frontier, but behind the Vistula and the armies, once concentrated, advanced from the Niemen, west of Kovno, from Warsaw, from Brest-Litovsk on the Bug and from the Rowno-Dubno-Lutsk fortresses west of Kiev. Thus the military as contrasted with the political frontier of Russia was behind the Vistula, the Niemen and the Bug.
The Austro-German plan contemplated a defensive fight on the north, in East Prussia, and an offensive campaign from the south, aimed at Lublin and Brest-Litovsk. The Russians on their side planned an immediate invasion of East Prussia from Warsaw and Kovno and a far more considerable offensive into Galicia from the Rumanian boundary to Rowno. The objective of the northern operation was the conquest of the whole of Prussia east of the Vistula, that of the southern the capture of Lemberg and the conquest of all Galicia. Combined, these two movements would abolish the Polish salient and give the Russian right flank the protection of the Baltic, the left the cover of the Carpathians. Only then could there be any safe advance by the center through Poland upon Posen and Breslau and thence upon Berlin.
Russian mobilization was more rapid than Russia's allies could have hoped for and it wholly confounded the Germans. While the Battle of the Marne was still two weeks off Russian forces were sweeping west from the Niemen and approaching Königsberg, a second army was striking north from Warsaw. East Prussian populations were fleeing before the invaders and a German disaster seemed imminent.
The genius of Hindenburg, who now appeared upon the eastern battle ground, saved the situation. Gathering in all his available forces and leaving the Russian army coming from the Niemen almost unopposed, he caught the Warsaw army in the swamps about the frontier in the last days of August and, thanks to his generalship and heavy artillery, destroyed a Russian army. Tannenberg was a great victory and it saved East Prussia. The Niemen army had to retreat rapidly to escape destruction. At the time, it was asserted that the Russian invasion had compelled the Germans to draw upon their western front to meet the thrust and thus to weaken their armies in advance of the decisive battle. This is not now believed to be true, but there is no doubt that it drew reserves who might otherwise have gone to the west or to the south.