Not cheaply were these spectacular triumphs won by the advancing armies of Mackensen and the Archduke Joseph Ferdinand. Their troops had been twice thrown back on the Dniester before that river had been finally won—a passage which cost them, all told, some 150,000 men; and in the successive retreats which followed, the Russian infantrymen turned repeatedly on their pursuers to prove that they were still capable of enforcing a price for every yard of ceded territory.
Mackensen's 'drive' was only part of the German Higher Command's plan for destroying the Russian armies in 1915. While the Austro-German phalanx was thus thrusting its way towards the Lublin-Cholm railway line, a simultaneous movement was in progress in the north, which had for its first objectives the great fortresses of the Polish salient, and Warsaw itself. Here Hindenburg, who was still in supreme command of the Austro-German forces on the Eastern front, had no longer General Ruzsky opposing him, Ruzsky having handed over the Russian northern command to Alexieff owing to ill-health. No matter how bravely the Russian infantrymen fought, or how ably they were led, they could not stand up against the hurricane of shot and shell which now blasted a path for the fully-equipped armies of von Below, von Eichhorn, von Gallwitz, von Scholtz, Leopold of Bavaria, and von Woyrsch. Prasnysz was won by von Gallwitz after a fierce battle in the middle of July, the Russians retiring to the shelter of the Narew fortresses guarding Warsaw from the north-east. Mackensen's advance to the south was resumed the next day, and the Grand Duke Nicholas, foreseeing the peril of this double threat, realized that his only hope lay in flattening the Warsaw salient and thus shortening his line. This sealed the fate of Warsaw, which was entered by Prince Leopold of Bavaria on 4th Aug. Ivangorod fell on the following day; Kovno on the 17th; Novo Georgievsk on the 19th. These losses, though deplorable, were not vital while the Russian armies still retained power to retaliate and recoup. Hindenburg strained every nerve to crush them once and for all. Ossowiec fell on 22nd Aug., the Russians resuming their retreat from the Niemen and Bobhr. Brest-Litovsk had already been threatened by the converging movement of Mackensen and Prince Leopold of Bavaria. Seeing no hope of saving it in the face of the continued pressure, the Grand Duke Nicholas evacuated this most easterly of the Polish quadrilateral of forts on the 25th, having previously stripped it, as in the case of the other evacuated fortresses, of all war material.
Hindenburg strove to complete the discomfiture of the main Russian armies by a fresh advance on his extreme left, where von Below was ordered to push through Courland towards Riga, with Petrograd as the ultimate goal in the following year. On this front the Russians had already been forced to relinquish Memel, just across the German frontier, as well as Libau. German naval forces had shared in the operations on the Riga coast-line, and when von Below, after carrying Mitau, some 30 miles from Riga itself, met with prolonged resistance, they made an ill-fated attempt to capture the port from the sea. This was on 18th Aug., when the Russian fleet appeared on the scene while the German naval contingents were attempting to land in flat-bottomed barges at Pernau. The landing forces were annihilated, and the German ships beaten off with a loss of two cruisers and eight torpedo-boats. The Russians only lost an old gunboat in this one-sided action.
The naval operations, however, had little effect on the main issue. Russian fortresses continued for another month to fall like ninepins before the Austro-German armies. Grodno was evacuated at the beginning of September, and though General Everts escaped from Brest-Litovsk with his supplies and guns, he could not hold up Mackensen's irresistible march on Pinsk, even in the Pripet marshes, which were dry at that season of the year. Pinsk was occupied on 16th Sept. Nowhere was the pressure relaxed. In the south, where the flood-tide of the Teutonic advance had never set so strongly, the attack on the Volhynian fortresses had been vigorously opposed by Ivanoff; but Boehm-Ermolli entered Lutsk on 1st Sept., and the Austrians recaptured Brody on the same day.
The vital blow at this stage was being delivered in the north, where von Below, bent on reaching Riga for his winter quarters, was marching on the Dvina lines with the immediate object of crossing that river and turning the whole Russian front as far as Dvinsk. The extreme left flank of the Germans fought desperately for the Dvina crossing at Friedrichstadt, but failed to make it good, and the danger-point shifted towards Vilna, the ten days' battle for which was decided at Meiszagowla on 12th Sept. Though two Russian divisions of the Imperial Guard were brought up to defend this key position, they were powerless to hold it against the great weight of German artillery. With its capture on 12th Sept., Vilna's fall became merely a matter of days. Before the Vilna armies could make good their escape, Hindenburg endeavoured to crown his triumph by outflanking them on both sides, von Eichhorn's cavalry sweeping round from Vilkomir in the north, and von Scholtz pressing forward, though less rapidly, on the southern side of the salient.
In this supreme crisis on the Eastern front, Ruzsky, recovered from his illness, returned to his command of the northern battlefields, and signalized his reappearance—not for the first time—by changing the whole complexion of affairs. Reinforcements enabled him in the first place methodically to evacuate the Vilna salient under their protection. Hindenburg endeavoured to counter this by rushing up cavalry reinforcements, with 140 guns, to support his outflanking thrust in the north, which, reaching Vidzy on the 16th and Vileika on the following day—this being well to the rear of the Vilna armies—threatened irremediable disaster to the retreating Russians. They were saved by the series of flank-guard battles securing their one avenue of retreat, and Ruzsky's counter-offensive from Dvinsk—a stroke so effective that the long German cavalry arm was in itself now in danger of being cut off. Vidzy was recaptured on the 20th; Smorgon, south of Novo Grodek, on the 21st; and Vileika before the end of the month.
On 15th Sept. Lord Kitchener had publicly declared that the Germans had "shot their bolt" on the Eastern front; and, so far as the immediate destruction of the Russian army as a force in being was concerned, this was true, though it was hard to believe while the wide sweep of the German advance was in full force. Ivanoff's reaction was equally marked in the south, where Brussiloff and Lechitsky took von Bothmer and Pflanzer-Baltin by surprise. Before the end of September the Austro-Germans had not only been pushed back to the Strypa, but had also lost both Dubno and Lutsk (23rd).