The forces engaged in this titanic conflict were the Russian Tenth Army, consisting, according to the Russian version, of four corps, under General Baron Sievers, and the German East Prussian armies, under General van Eichhorn, operating on the north on the line Insterburg-Lötzen, and General van Bülow on the line Lötzen-Johannisburg to the south of Van Eichhorn. Sources favorable to the Allies represent the strength of General Sievers's army as 120,000 men. They assert that the total German force consisted of nine corps, over 300,000 men. These are said to have included the Twenty-first Corps, which had been with the Crown Prince of Bavaria in the west; three reserve corps, also from the west; the Thirty-eighth and Fortieth Corps, new formations, from the interior of Germany; the equivalent of three corps from other sections of the eastern front; and a reserve corps of the Guard. The German official description of the battle credits the Russians with having had in this sector of the battle front in East Prussia at the beginning of February six to eight army corps, or about 200,000 men.
For months the heavy fighting in the east had centered on other sections of the immense battle line, running from the Baltic to the Carpathians. The second general Russian offensive, the great forward thrust of the Grand Duke Nicholas toward Cracow in the direction of Berlin, aimed through the center of the German defense, had been met, and the German counterthrust toward Warsaw had come to a standstill in the mud of Poland and before the stone-wall defensive of the Russians on the Bsura and the Rawka. Attacks launched by the Russians against the East Prussian frontier, centering at Lyck, in January, 1915, seemed to forebode a fresh Russian offensive intended to sweep back the German armies in this section whose position on the Russian right wing was a continual threat to the communications of the Russian commander in chief.
The Germans, disposing of comparatively weak forces, estimated at three army corps, were compelled to yield a strip of East Prussian territory, and had fallen back to positions of considerable natural strength formed by the chain of Mazurian Lakes and the line of the Angerapp River. They reported their forces standing on the defensive here as 50 per cent Landwehr, 25 per cent Landsturm, and only 25 per cent other troops not of the reserve. Repeated attempts of the Russians to gain possession of these fortified positions had, however, broken down. They had been directed especially against the bridgehead of Darkehmen and the right wing of the German forces in the Paprodtk Hills. Wading up to their shoulders in icy water, the hardy troops of the Third Siberian Corps had attempted in vain to cross the Nietlitz Swamp, between the lakes to the east of Lyck.