At the same time that these two groups formed a junction Von Mackensen came up with his forces from the south, taking Zamost and Lublin and investing Ivangorod. Immediately the drive for Warsaw began from all sides. Pultusk, on the Nareff, fell on July 25, 1915, and on July 30, 1915, the Russians began the evacuation of Warsaw and retreated toward a very strongly fortified line that had been prepared and ran from Kovno south through Grodno and Brest-Litovsk. CHAPTER XX THE FALL OF THE NIEMEN AND NAREFF FORTRESSES The 5th of August, 1915, was a fateful day for the Russian armies. The fall of Warsaw, on that date, was confirmed by the occupation of Poland's ancient capital by German forces under the command of Prince Leopold of Bavaria, brother of King Ludwig III of Bavaria and son-in-law of Emperor Francis Joseph of Austria-Hungary. This in itself would have been a severe setback to the Russian arms. But the consequences which this event was bound to have were of even greater importance. In an earlier part of this work we heard at some length of the arrangement of Russia's girdle of fortresses which--to repeat only the most important--stretched from Kovno in the north through Oliha, Grodno, Ossovetz, Lomza, Osholenka, and Novo Georgievsk to powerful Warsaw and from there to the south and east to Ivangorod and Brest-Litovsk. These permanent fortifications were supported by strong natural barriers or obstacles in the form of rivers. The Niemen, Bobr, Nareff, Vistula and Bug, with their interminable windings, made more difficult to cross in some places by extensive swamp lands, had, together with the fortified places, offered ideal means for strong defense. Again and again, throughout the first thirteen months of the war, German and Austrian troops had driven the Russian forces back to these defensive lines--but no farther. Behind this shelter the Russians were able to recuperate from the severest reverses and, thanks to a very extensive and comparatively scientific network of railways, reserves and reenforcements could be brought up from interior points until armies which apparently had been beaten to a standstill emerged again, stronger than ever in number and equipment, to undertake a new offensive against the German masses.