August von Mackensen [on white horse]
On 2 November 1914 Mackensen took over command of the Ninth Army from Hindenburg, who became Supreme Commander East (Oberbefehlshaber Ost). On 27 November 1914 Mackensen was awarded the Pour le Mérite, Prussia’s highest military order, for successful battles around Łódź and Warsaw.
By April 1915 the Russians had conquered all of western Galicia, the Austro-Hungarian slice of partitioned Poland, and were pushing toward Hungary. In response to desperate pleas the German supreme commander Erich von Falkenhayn agreed to an offensive against the Russian flank by an Austro-German Army under a German commander. The reluctant Austro-Hungarian supreme command agreed that the tactful Mackensen was the best choice for commanding the coalition army. Army Group Mackensen (Heeresgruppe Mackensen) was established containing a new German Eleventh army, also under his command, and the Austro-Hungarian Fourth Army. As chief of staff he was assigned Hans von Seeckt, who described Mackensen as an amiable, “hands-on commander with the instincts of a hunter.”  His army group, which had an overwhelming advantage in artillery, smashed through the Russian lines between Gorlice and Tarnow and then continued eastward, never giving the Russians time to establish an effective defense, retaking most of eastern Galicia by recapturing Przemyśl and Lemberg The joint operation was a great victory for the Central Powers, they had advanced 310 km (186 mi), and soon thereafter the Russians pulled out of all of Poland.
For a variety of reasons, the Russian army in summer 1915 was unable to handle reserves with the same kind of flexibility as the Germans in the west. It was partly a simple problem of numbers: there were not enough troops both for the front and for reserves. Indeed, resistance to break-through and resistance to exploitation of it demanded almost contradictory solutions, or so it seemed. The break-through could only be prevented if there were a ‘thick front line’, i.e. sufficient fire-power in the first position to defeat enemy penetration. But if the front lines were thickly-held, there would be little to spare for reserves. In eastern conditions, a break-through was generally more baneful in its consequences than in the west. Since there were fewer railways, and particularly fewer roads, and since weather-conditions often created mud, it was difficult for defenders to retreat with any speed. If their line were broken through, then troops to right and left of the break-through area could be out-flanked and surrounded before they could withdraw—hence the large number of prisoners that distinguished this front. The only way of resisting seemed to be the ‘thick front-line’ principle, to which the Russian army, and the other two, adhered for most of the war. Consequently, the proportion of troops used by the Russian army as reserves was much smaller than in the German or French case. To create reserves, it was necessary to withdraw troops from the front-line of a passive theatre, with all the complications of rivalry between the front-commands that this involved.
Furthermore, the functioning of railways on the Russian side was much less efficient than on the German, so that even when reserves did slip through the jealous clutch of a front-command, they could not be shifted rapidly. The assembly of IX Army in the Bukovina, in May 1915, appears to have taken a month, as had the assembly of XII Army on the borders of East Prussia in February. Even in 1916, transfer of a single corps of two divisions took twenty-three days or, if complete priority were given, a fortnight. This had little to do with the actual speed of trains—even slow trains could cover the whole length of the front from Vilna to Cherson in three or four days, if the journey were not interrupted. It occurred because of bottle-necks in rolling-stock: not enough could be freed at one time for the movement of a division in four days. The contrast with German handling of reserves were plain, for all to see. When the Germans shifted their IX Army early in November from just north of Cracow to their jump-off points around Toruń for their Lódz offensive, they sent four army corps in five days, with 800 troop-trains, and had performed much the same feat when they set up IX Army late in September. In the last ten days of April, using the relatively poor lines south of Cracow, they sent eight infantry divisions to help the Austrians, with sixty trains daily; and similarly took only four days to send three divisions from Ludendorff’s front in East Prussia to southern Hungary—174 trains, at forty-four per day. More significantly, when the Germans were actually in occupation of the railway-lines used by the Russian army in 1914–15, their exploitation of them was much more efficient, even though problems of wartime devastation were complicated by those of the different Russian gauge. In the first week of July 1916, to counter the Russian break-through on the Styr, they sent 494 troop-trains, with ten divisions, and ninety-eight artillery trains on lines to Kowel, Cholm and Vladimir-Volynski that in Russian days were supposed to be painfully undeveloped.
The reasons for this immeasurably superior German performance lay in a combination of misfortune and mismanagement. The quality of labour mattered; and in any case there were always fewer railwaymen in Russia before 1914 than in Germany, although the mileage to be covered was greater and the technical problems more demanding. The army had gone to war with 40,000 men in its railway-battalions, and of these, over a third were wholly or partly illiterate, while three-quarters of the officers had had no technical training. Although the railway-troops did expand, to 200,000, lack of training was always a serious obstacle. Furthermore, the various instances of command did not sort out their priorities. Ronzhin, at Stavka, had no assistants at all, beyond his two subordinates and a clerk, sitting in half of a railway-carriage in Baranovitchi. The front railway-directors were in fact supreme, but they too had a limited view of their job, and arranged things without reference to each other. The military, who controlled a third of the country’s railway-mileage and over a third of its rolling-stock, also quarrelled with civilians in the ministry of transport, itself run none too efficiently, and an amount of rolling stock fell, unused, between these instances.
But the heart of the railway-problem, at least at the front, was the horse. The Russian army maintained a constant million horses, partly because only horses could overcome the local transport-problems of eastern Europe, partly because the army remained faithful to cavalry divisions long after other armies had abandoned them. Grain was by far the bulkiest item for railways to transport. In December 1916, the army’s daily needs amounted to sixty trains, of fifty waggons each. Supplies for the soldiers amounted to about 16,000 tons of flour, grits, fat, salt, sugar, preserved meat, and this took 1,095 waggons daily. Supplies for horses took 1,850 waggons daily, for 32,000 tons of barley, oats, hay and straw. Before an offensive, with a great gathering of cavalry, demand for fodder rose even higher. Ivanov, in December 1915, demanded, for his Bessarabian offensive, 13,700 tons of salt, sugar, grits; twelve million portions of preserved meat; 180,000 tons of fodder. The first two, designed for the men, needed 667 waggons; the last, for the horses, 11,385. The contrast with the German army was plain. Between 15th June and 15th November 1917, the German IV Army, facing the British offensive in Flanders, took in all 6,591 trains, of which 3,942 carried troops, 1,854 artillery and 795 supplies for men and horses of all kinds. This broke down into 242,185 waggons, of which 51,481 counted as supply of all types. In other words, the horse, which required about a fifth of German transport, needed more than half of Russian transport. Not surprisingly, the movement of reserves behind Russian lines was much slower than behind German lines, because the rolling-stock was preoccupied with carriage of supply for horses. This was, to some degree, a consequence of the nature of the front, with its poor roads. But it also reflected a belief in cavalry that cavalry did little to justify, although there were, including Cossacks, some fifty cavalry divisions in the Russian army at a time when all other armies had converted their cavalry divisions to ‘mounted infantry’, i.e. dismounted cavalry.