Battle of Tannenberg.
Hindenburg's general strategical plan was as simple as the carrying out of it, considering the means at his command was difficult. Facing him were two armies still out of contact with each other, or at least only very loosely connected. Each alone outnumbered him at least by 50,000; combined they were more than three times as powerful as all his forces. His only hope, therefore, was in attacking them separately. Thus he chose to strike first at Samsonoff's army which was much farther spread out than Rennenkampf's, and would find it much more difficult than the latter to keep open its main line of retreat and supply. Its left rested on Soldau, its right on Frankenau, while its center had been pushed forward to Allenstein through Soldau, and southeast from it ran the only direct railroad to his Polish base by way of Mlawa. Three other lines centered there, one in the west from Thorn, one in the northwest from Eylau (connecting with Danzig and Königsberg), and one in the east from Neidenburg, which from there run north to Allenstein and northeast to Johannisburg and Lyck. Apparently centering his efforts on pushing his advance, Samsonoff had neglected to secure the former two roads.
On August 26, 1914, Von Hindenburg occupied both and took Soldau Junction. The shortest line of retreat had now been cut off to the Russians, whose forces were scattered over a considerable territory, and on account of lack of railroads could not be concentrated quickly or efficiently at any one point. Though a determined effort was made on August 27, 1914, to retake Soldau, it was foredoomed to failure. Samsonoff's left was thrown back on Neidenburg, making his front even more unwieldly than before.
At this time the German front was very short, its left being at Hohenstein, about halfway between Soldau and Allenstein and slightly northeast of Tannenberg. But it made up in activity what it lacked in length. In vain the Russians tried to break the German ranks and open up a road to the northwest. Much blood was spilled on both, sides during three days' fighting, but the German line held. In the meantime the Russians had evacuated Allenstein, feeling the imperative need of shortening their front. This gave Von Hindenburg the railroad that ran almost parallel to the Russian front as well as the splendid main road that runs alongside of it. Commandeering every available motor vehicle from the entire surrounding countryside, he immediately extended his line and swung around the Russian right as previously he had swung around their left. Almost every road, rail or otherwise, that was of any importance was now in the hands of the Germans and along them could be sent men and guns with overwhelming rapidity. With relentless energy Von Hindenburg now used his intimate knowledge of the territory in which he was fighting. Wherever he knew the most hopeless territory to be, there he drove the Russians. Mazurian swamps and lakes did all that he had ever claimed they would do and more. They swallowed up his enemy by the thousand, they engulfed his guns and sucked in his horses.
Within a week after Von Hindenburg had reached East Prussia the problem of the Narew Army had changed from how to extend its advance most quickly to how to escape from this bottomless pit along the few inadequate lines of escape that were left. The morale of this Russian army was broken. For even the most stolid Russian peasant soldier, whom neither the roar of guns nor the flash of bayonets could move, quaked at seeing whole companies and batteries disappear, in less time that it takes to tell about it, in the morasses of a country without firm roads and a minimum of solid ground.
On the last day of August, 1914, thousands of Russians had laid down their arms and were sent back into central Germany. Of Russian armies of more than a quarter of a million nearly a hundred thousand fell into German hands. Almost half as many more were killed or wounded. The Russian commander in chief was killed on August 31, 1914. Only one corps escaped by way of Ortelsburg and Johannisburg, while scattered fragments of varying size fought their way out, some into north Poland and some into the protecting arms of the Niemen Army. Most of the guns of Samsonoff's army were either captured by the Germans or lost in the swamps. This one week's battle among the Mazurian lakes is known now as the Battle of Tannenberg, so named after a small town west of and halfway between Soldau and Allenstein.
Without giving his troops any rest Von Hindenburg now turned against Rennenkampf's forces. But, in spite of the rapidity of movement, the German commander could not accomplish all that he had set out to do. Apparently his plan was now to strike north past Angerburg and Goldap to Gumbinnen, or possibly even to Eydtkuhnen in order to cut off the retreat of the army of the Niemen and drive them in a southerly direction to their destruction in the Mazurian lakes, just as he had done in his easterly drive against the Narew Army. But Rennenkampf was too quick. He recognized the danger that threatened him through the defeat of Samsonoff's forces and he began his retreat as soon as it became evident that the other army's cause was lost. He was in a much more advantageous position than his colleague had been. For not only did the territory through which he had to fall back offer no particular difficulties when once he had escaped Hindenburg's attempt to push him up against the Mazurian lakes, but he had also a fairly efficient network of railroads at his command centering in Insterburg.
Long before he evacuated this city on September 11, 1914, he had drawn in most of his outlying formations in the north and west and had sent them back safely across the border and behind the protection of the Niemen and its shield of fortresses—Kovno, Olita, and Grodno. In this he was also materially assisted by the stubborn resistance which Von Hindenburg encountered at Lyck at the hands of a small army that had been sent out from Grodno to aid him, and the nucleus of which consisted of an entirely new Finnish, and an equally complete, Siberian Corps. In spite of this, however, the pressure of the victorious Germans was strong and rapid enough to force him to a generally hurried retreat. The losses in killed and wounded were comparatively small, for almost all the fighting was rear-guard action. But the Germans succeeded in gathering in about 30,000 more prisoners, chiefly detachments that had been unsuccessful in connecting in time with the main army. Much more serious was the loss of some 150 guns and vast quantities of war material for the removal of which both time and means had been lacking.
On September 15, 1914, Von Hindenberg could announce that the last of the invaders had either been captured or driven back and that not an acre of German soil was in the possession of the Russian forces. On that date, moreover, he had already advanced far enough into Russian territory to occupy the seat of government of the Russian province of Suwalki, almost 150 miles in direct line east of Tannenberg, though less than 20 from the German border. From that point on he intrusted the further conduct of these operations to Lieutenant General von Morgen, who had been one of his division commanders at Tannenberg.
By September 23, 1914, Rennenkampf had completed his retreat behind the Niemen. The fighting which took place during the ensuing week is commonly designated as the "Battle of Augustovo," though it covered a much larger area. Augustovo itself is a small town about ten miles from the German frontier, about twenty miles south of Suwalki, and forty miles northwest from Grodno.