But should this railroad be taken by the Germans, the advantage would immediately be reversed. And if once the Russian lines were driven back beyond the railroad, a division of their forces would be forced upon them; their armies would be obliged to group themselves beside the three east and west branches already mentioned, for only by these three systems could their forces be supplied, lateral communications being absolutely lacking. And this is the key to the fighting, not only in the northern section of the front, but all along the line, down to Galicia. Naturally, only the Russian railroads need be considered, for in the first months of the war the Germans are the invaders in the northern half of the eastern front, except for a few short periods in the beginning. Compared to the German railway lines near the frontier, the Russian lines are very few. There are two distinct railway lines running from Germany into East Prussia, with innumerable branches leading to all points of the Russian frontier, laid especially for military purposes. It was along these that we shall witness the German forces rushed from Belgium to drive back the first Russian advance. But, of course, the moment the Germans enter Russian territory they have no advantage over the Russians, since even their wonderful efficiency does not enable them to build railroads as fast as an army can advance. Hence, we observe their efforts to gain possession of the Russian railroads. We come now to the central part of the eastern front. Here, just below East Prussia, Russian Poland projects into German territory in a great salient, about 200 miles wide and 250 long, resembling a huge bite in shape. This land is a monotonous, wind-swept plain, slightly undulating, its higher parts not even 500 feet above sea level. To the northward and eastward it descends gradually into the still lower lands of East Prussia and White Russia, but in the south it lifts into the foothills of the Carpathian Mountains. Gigantic armies are to move over this plateau, timbered in parts with oak, beech, and lime, and in some sections deeply cut by small rivers and streams forming fissures, some narrow and craggy, others broad and sloping with marshy bottoms. Toward the south the soldiers must cross narrow ravines in all directions, often covered with wild, thick undergrowth. The chief river is the Vistula, which enters by the southern boundary and flows first north, then northwest, skirting the plateau region at a height of 700 feet, finally making its exit near Thorn, thence on to the Baltic through East Prussia. Its valley divides the hilly tracts into two parts: Lublin heights in the east and the Sedomierz heights to the westward. Picture in your mind the great armies approaching these ridges, the most notable of which is the Holy Cross Mountains, rising peaks almost 2,000 feet above sea level.