On the Eastern Front, greater distances and quite considerable differences between the equipment and quality of the opposing armies ensured a fluidity of the front that was lacking in the west. Trench lines might form, but to break them was not difficult, particularly for the German army, and then mobile operations of the old style could be undertaken.

Urged by the French to take offensive action against the Germans, the Russian commander in chief, Grand Duke Nicholas, took it loyally but prematurely, before the cumbrous Russian war machine was ready, by launching a pincer movement against East Prussia. Under the higher control of General Ya.G. Zhilinsky, two armies, the 1st, or Vilna, Army under P.K. Rennenkampf and the 2nd, or Warsaw, Army under A.V. Samsonov, were to converge, with a two-to-one superiority in numbers, on the German 8th Army in East Prussia from the east and the south, respectively. Rennenkampf’s left flank would be separated by 50 miles from Samsonov’s right flank.

Max von Prittwitz und Gaffron, commander of the 8th Army, with his headquarters at Neidenburg (Nidzica), had seven divisions and one cavalry division on his eastern front but only the three divisions of Friedrich von Scholtz’s XX Corps on his southern. He was therefore dismayed to learn, on August 20, when the bulk of his forces had been repulsed at Gumbinnen (August 19–20) by Rennenkampf’s attack from the east, that Samsonov’s 13 divisions had crossed the southern frontier of East Prussia and were thus threatening his rear. He initially considered a general retreat, but when his staff objected to this, he approved their counterproposal of an attack on Samsonov’s left flank, for which purpose three divisions were to be switched in haste by rail from the Gumbinnen front to reinforce Scholtz (the rest of the Gumbinnen troops could make their retreat by road). The principal exponent of this counterproposal was Lieutenant Colonel Max Hoffmann. Prittwitz, having moved his headquarters northward to Mühlhausen (Młynary), was surprised on August 22 by a telegram announcing that General Paul von Hindenburg, with Ludendorff as his chief of staff, was coming to supersede him in command. Arriving the next day, Ludendorff readily confirmed Hoffmann’s dispositions for the blow at Samsonov’s left.

Meanwhile, Zhilinsky was not only giving Rennenkampf time to reorganize after Gumbinnen but even instructing him to invest Königsberg instead of pressing on to the west. When the Germans on August 25 learned from an intercepted Russian wireless message (the Russians habitually transmitted combat directives “in clear,” not in code) that Rennenkampf was in no hurry to advance, Ludendorff saw a new opportunity. Developing the plan put forward by Hoffmann, Ludendorff concentrated about six divisions against Samsonov’s left wing. This force, inferior in strength, could not have been decisive, but Ludendorff then took the calculated risk of withdrawing the rest of the German troops, except for a cavalry screen, from their confrontation with Rennenkampf and rushing them southwestward against Samsonov’s right wing. Thus, August von Mackensen’s XVII Corps was taken from near Gumbinnen and moved southward to duplicate the planned German attack on Samsonov’s left with an attack on his right, thus completely enveloping the Russian 2nd Army. This daring move was made possible by the notable absence of communication between the two Russian field commanders, whom Hoffmann knew to personally dislike each other. Under the Germans’ converging blows Samsonov’s flanks were crushed and his centre surrounded during August 26–31. The outcome of this military masterpiece, called the Battle of Tannenberg, was the destruction or capture of almost the whole of Samsonov’s army. The history of imperial Russia’s unfortunate participation in World War I is epitomized in the ignominious outcome of the Battle of Tannenberg.

The progress of the battle was as follows. Samsonov, his forces spread out along a front 60 miles long, was gradually pushing Scholtz back toward the Allenstein–Osterode (Olsztyn–Ostróda) line when, on August 26, Ludendorff ordered General Hermann von François, with the I Corps on Scholtz’s right, to attack Samsonov’s left wing near Usdau (Uzdowo). There, on August 27, German artillery bombardments threw the hungry and weary Russians into precipitate flight. François started to pursue them toward Neidenburg, in the rear of the Russian centre, and then made a momentary diversion southward, to check a Russian counterattack from Soldau (Działdowo). Two of the Russian 2nd Army’s six army corps managed to escape southeastward at this point, and François then resumed his pursuit to the east. By nightfall on August 29 his troops were in control of the road leading from Neidenburg eastward to Willenberg (Wielbark). The Russian centre, amounting to three army corps, was now caught in the maze of forest between Allenstein and the frontier of Russian Poland. It had no line of retreat, was surrounded by the Germans, and soon dissolved into mobs of hungry and exhausted men who beat feebly against the encircling German ring and then allowed themselves to be taken prisoner by the thousands. Samsonov shot himself in despair on August 29. By the end of August the Germans had taken 92,000 prisoners and annihilated half of the Russian 2nd Army. Ludendorff’s bold recall of the last German forces facing Rennenkampf’s army was wholly justified in the event, since Rennenkampf remained utterly passive while Samsonov’s army was surrounded.

Having received two fresh army corps (seven divisions) from the Western Front, the Germans now turned on the slowly advancing 1st Army under Rennenkampf. The latter was attacked on a line extending from east of Königsberg to the southern end of the chain of the Masurian Lakes during September 1–15 and was driven from East Prussia. As a result of these East Prussian battles Russia had lost about 250,000 men and, what could be afforded still less, much war matériel. But the invasion of East Prussia had at least helped to make possible the French comeback on the Marne by causing the dispatch of two German army corps from the Western Front.

Having ended the Russian threat to East Prussia, the Germans could afford to switch the bulk of their forces from that area to the Częstochowa–Kraków front in southwestern Poland, where the Austrian offensive, launched on August 20, had been rolled back by Russian counterattacks. A new plan for simultaneous thrusts by the Germans toward Warsaw and by the Austrians toward Przemyśl was brought to nothing by the end of October, as the Russians could now mount counterattacks in overwhelming strength, their mobilization being at last nearly completed. The Russians then mounted a powerful effort to invade Prussian Silesia with a huge phalanx of seven armies. Allied hopes rose high as the much-heralded “Russian steamroller” (as the huge Russian army was called) began its ponderous advance. The Russian armies were advancing toward Silesia when Hindenburg and Ludendorff, in November, exploited the superiority of the German railway network: when the retreating German forces had crossed the frontier back into Prussian Silesia, they were promptly moved northward into Prussian Poland and thence sent southeastward to drive a wedge between the two armies of the Russian right flank. The massive Russian operation against Silesia was disorganized, and within a week four new German army corps had arrived from the Western Front. Ludendorff was able to use them to press the Russians back by mid-December to the Bzura–Rawka (rivers) line in front of Warsaw, and the depletion of their munition supplies compelled the Russians to also fall back in Galicia to trench lines along the Nida and Dunajec rivers.